Smut Writing Tips for Aspiring Writefags

Spelling and Grammar:
Remember to keep a close eye on both spelling and grammar- while your average spellchecker (like in Word) can catch most obvious mistakes, it's not perfect and things can still slip through the cracks. While this may sound like nitpicking, these errors are an irritating distraction at best and at worst can completely change the meaning of a sentence in a way that is neither intended nor desirable (e.g. saying "her penis" when you meant to say "his penis"). A good rule of thumb is to read your work out loud to yourself before submitting it- some mistakes are easier to notice when you're hearing it being read instead of just looking at the words.

Plot and Porn- How Much is Enough?:
While too much plot can be a bit of a boner-killer, jumping straight into the action isn't always the best approach either; depending on the story length, 3-5 intro paragraphs is usually enough. (Remember, you are writing a fapfic, not a novella.) You can add more if it's going to be part of a series instead of a one-shot, mind you. Speaking of which, making a fic into a series is something best left for more experienced writers, since if you make a fic into a series you have to deal with the additional problems of continuity and creating an overall plot that ties the individual chapters together, not to mention the increased importance on developing the characters and the need to plan the story ahead of time.

Under no circumstances should you resort to having a character in a sex scene be a nameless bit character. If they don't have a discernible personality and defining features, they shouldn't be in the story to begin with.

Regarding your Female Audience (Yes, They Exist):
It's not just about what exactly is happening- they don't need to know where where all the hands are at all times and what everyone is doing in each scene. We have porn for that. Erotica and smut for women are more about the emotions of the moment and what everyone is feeling rather what their dicks/pussies/arseholes are feeling. Anyone can write "he stuck his dick in her pussy and they had sex" or some variation of that phrase, but it takes real skill to convey both the physical sensations and the emotional states of the people involved in the scene.

On Language:
Don't use excessively vulgar slang like "cunt" or "twat". Instead, use terms that would be more appropriate for the setting the story takes place in. And don't use the same word too often, it gets repetitive after a while. (To quote Quest Thread General, "Using the word ecstasy more than once a paragraph when writing smut makes you seem like a massive tool or a fifty-year-old British lady, and either way the reader will want to gut you.") Some purple prose is alright, but too much is never good (it makes it difficult to figure out what's happening and at worst can make what was supposed to be an arousing scene unintentionally humorous instead). Just try to read any of the sex scenes in Fifty Shades of Grey without snickering and you'll see what I mean.

Your Fetish is Not Always My Fetish:
While these threads are quite tolerant of many different forms of /d/eviancy, you can't just assume that everyone will automatically share your kinks. Because of this, it's always a wise idea to say what's going to be in your fic before posting it on the board, if only because some readers do not like surprises and would prefer to avoid stories of one type or another. Nobody is stopping you from posting your story about furry dickgirls, but please be considerate of others and mark it as such when you post it. (That said, there are some fetishes which are almost certain to be rejected- in particular, you shouldn't even think of asking about NTR unless you want the entire board to turn against you.)

Speaking of which, posting the story right on the thread itself is a bad idea, particularly if it's a long one- it eats up the post count and might get the mods mad if it contains explicit material. Post it in a pastebin instead and link the thread to it instead. Same goes for lewd artwork; at the very least, put a censor bar over any exposed nipples, dicks, pussies, and/or assholes if you wish to post it directly in the thread- spoilering it won't always spare you from the mods' wrath, so err on the side of caution.

Do Your Research:
If you want to write about an established setting (e.g. Warhammer 40k), doing a bit of reading about it is strongly advised- if you want to see what happens when your story takes too many creative liberties with the canon it's derived from, read one of C.S. Goto's books (or better yet, don't read them- they aren't universally reviled on /tg/ for nothing). 1d4chan is a good source of info when it comes to answering most of your questions (and it's a good idea to check the Stories category to learn from previous writers' works). Other wikis such as Lexicanum can also be useful, and when in doubt you can always ask /tg/ for help.

The Act Itself:
“Giving a reader a sex scene that is only half right is like giving her half a kitten. It is not half as cute as a whole kitten; it is a bloody, godawful mess. A half-good sex scene is not half as hot; it actually moves into the negative numbers, draining any heat from the surrounding material.” -Sandra Newman, How Not To Write a Novel

Try varying the length of your sentences and paragraphs to reflect what's going on. The foreplay should be slow, flowery and described in long chunks. By contrast, the paragraphs describing the actual sex act should be shorter and more succinct, and the sentences that constitute it should be equally short and to the point. A good example of this at work can be seen in Smutomancer's "The Covenant of Sanctified Souls" and "Avelorn Bliss".

If you're going to use euphemisms in place of the more common terms, be sure that your readers can figure out what it's supposed to mean. Consider the following line from the infamous fanfic My Immortal: "He put his thingy in my you-know-what and we did it for the first time." If it takes someone more than five minutes to determine what your characters are doing, then you should consider changing the wording to make things clearer. On the other hand, if you just describe it mechanically, the end result will be no more arousing than an IKEA instruction manual. (And on that note, there's no need to rattle off specific measurements- saying that a character has apple-sized breasts is fine, but saying that she has a 32A bust size tends to be a libido-killer.)

Pacing Tips:
As tempting as it might be to skip straight to the action, sometimes it's better to draw the tension out a bit. To quote Anonymous: " Some fapfics are like rockets, going up and popping all in one go. Some fapfics are like gasoline automobiles, starting right up and getting you where you need to go while still letting you see some scenery. Some are like diesel trucks- takes a long time to start and even longer to get going, but once it's up to speed there's no stopping it. All three have their uses and audiences, but for the long haul accept no substitute." That said, some styles work better for some people than others, so experiment and find out which one works best for you.

Back Your Shit Up!
You can write a veritable masterpiece, but if you forget to save it all of that work will be for absolutely nothing. Use Word or Open Office or some other word processor that lets you save your work, and get a flash drive to save it to just in case your computer gets a virus or something. While pastebin is certainly useful for hosting the work, don't type it up there directly- copypaste it from Word, lest you hit the F4 button and delete it all by accident. As Smutomancer put it, "Never fucking write in a web browser."

Who's Doing What?
While lesbians and gay stuff are appreciated just as much as any other kind of smut, it illustrates a major problem- the constant use of only one pronoun makes it very difficult to tell who does what to who. If you're going to do something like that, be sure that you make it perfectly clear which character is doing which actions. Nothing like confusion to kill a boner. (And it's even worse if the characters' names differ by only one or two letters, so keep the names distinct enough to remember which character is which.)

How to Punctuate Sentences: An Example in Action
Remember, after a piece of dialogue and right before the quotation marks, you need to put a comma.

"Like this," said Anon1.

"Do you mean like this?" asked Anon2.

"That's a different case," Anon1 continued, "but it works since the question mark is indicating that piece of dialogue is a question but the sentence is not. Which is why you have a period at the end of said sentence."

"That's really cool!" Anon2 replied happily.

"Indeed. But you did the same thing again, this time with an exclamation mark." Anon1 thought of how to continue, realizing that this sentence would work as an example of when you ARE supposed to end dialogue with a period, since the piece after it is not modifying it in any way, and being rather smug about it. "Try it again," Anon1 said, giving another example of how to do it properly.

"I don't think I can," Anon2 lamented in a sad tone.

"But you just did..." Anon1 was wary of using an ellipses in such a manner, but it was technically correct and he reasoned the example would be helpful.

"I did? Neat."

"Indeed."

"That was fun."

"I agree," said Anon1, giving an indication who was speaking after a succession of non-specified dialogue that was easy enough to follow without the usual identifiers, which is usually a good indicator that they are unnecessary in such cases. "I think we're done here," he concluded in an overly smug way like the jackass he really is.

An Aside on Futa
In case you have somehow never been on 4chan before, "futanari" (or futa for short) is a word denote women who happen to have penises (aka "dickgirls"). For reasons too complicated to explain here, this is a very common fetish- so common, in fact that /d/ was first made for the sole purpose of containing it all. However, this glut of material is not conducive to writing futa material for several reasons. To paraphrase one anon, "I don't mind the idea of a chick with a dick, but dear god are they so overplayed, it's not even /d/ to me anymore. Futa is BORING, futa is OVERPLAYED, FUTA HAS BECOME FUCKING VANILLA, it's like the first thought fetish writers have when it comes to women dominating/sexually aggressive towards men is "put a dick on her", SHOW SOME GODDAMN CREATIVITY FOR ONCE. And don't even get me started on how it's always a stupidly huge cock to show how much better they are at fucking than a guy." Quite simply, there is an entire board dedicated to futa material, so it's strongly advised that you consider putting a unique spin on the idea rather than just adding it for its own sake.

We Repeat, Back Your Shit Up!
This tip may be redundant, but it's worth repeating if only because of the potential risks of not following it.

More than one writefag has had much of their work undone in an instant because something got deleted and they didn't have a backup. You might think "I'm careful, I won't need a backup"- you'd be wrong. Computers die, hard drives fail, files get inadvertently erased, flash drives are saved over by that idiot roommate of yours who insists on "borrowing" the flash drive to fill it with pictures of his landwhale girlfriend- in short, shit happens. Save it to a relatively safe external source like Google Drive or the Pastebin website itself in addition to your own computer or flash drive. It's far less of a hassle than losing countless hours of time and effort to a freak accident with no hope of recovering them.