[Disclaimer: When I was a kid, I wrote like a kid. I still write like a little boy sometimes. Feel free to mock. :D]
How to Write Slash Like A Little Girl
Design one character
Make him the utterly cutest guy ever. Spend hours on his appearance, designing his hairstyle, eye color, and clothing style, drawing him with colored pencils on the back of your notebook. You don't have to develop a personality for him. Just use yours. Don't worry about coming up with his motivation, either; his goal is to achieve True Love, obviously.
Your best friend will design the other character. Make sure they don't have the same hair color.
Gay boys are shy and conflicted
You know this because you're shy and conflicted, and what are gay boys but little girls with extra treasure in their pants, right? Low self-esteem is a must; your character's self-loathing and your best friend's character's praise and adoration can keep the story going for dozens of chapters as long as your character never starts to believe what he hears.
Names are crucial
Make sure your character has a name that sounds lovely when crooned in the throes of passion. You're not sure what throes of passion sound like, but they're probably lyrical. Try to make the name unique and special, too, so that when you're super popular and famous you can Google it and find secret caches of fanart that your adoring fans were too shy to show you.
Now you and your best friend can take your characters out for a test drive. The point of the excercise is to elicit wibbles and aww's from your friend and create a warm feeling in your nethers. Careful, though! You might accidentally come up with an interesting plot, and those are hard work.
Drawing on your notebook and RP with your friend counts as preparation. Now you better start writing really fast before you lose your nerve. If you take the time to outline a plot or design settings or other characters, the mood might wear off!
You can bond with your readership by including lots of song lyrics. Any time the characters are listening to music, be sure to give the song title and who it's by. For extra credit, look up the lyrics and post them entire, then describe how they make the character feel.
The character is, of course, listening to the same music you are, and how he feels is how you feel.
It's important to describe things so the readers can visualize them. The two most crucial things to describe are: 1. attractive people's appearances, and 2. what they're wearing. Be sure you don't leave out a single freckle or accessory. Beautiful locations should also be described in vivid detail, so the reader can get alongside your design vision. Cute things like children or pets can be described or not at your discretion.
Don't describe anything boring or ugly. The reader doesn't want to see that.
You have to think up names for them, but that's about it. Other than that you can just copy stock characters from anime. No one will know the difference.
Use big words
It doesn't matter if you don't know what they mean. If they sound about right, throw them in wherever. It makes you sound smart. You can also make up new, bigger versions of existing words. If 'disoriented' is good, 'disorientated' is better!
Tell, don't show
Sure, they say it's the other way around, but you know the secret truth: making characters actually play out their conflicts and motivations in the course of action just takes waaaaaay too long. You might lose readers who get bored waiting for the snuggles. Just describe the character's personality and situation in a paragraph or two and get it out of the way.
If they ever act out of character later, just call it a strange feeling. Use this template: "He was usually so [X], but for some strange reason he felt [Y]."
Plot is an illusion
You do need a plot idea, but don't worry about actually following through. All the plot has to do is make it harder for the boys to get together for a while. Once they've declared their love, you can just brush off the rest of it.
Sex is a crucial part of the slash story. It's what everyone is waiting for. You can't just throw it in any old way, though. You don't want people thinking your adorable character is a slut! Fortunately, you'll be fine if you follow one simple rule: no consensual sex without a declaration of love. Once love is declared, sex comes next. Missionary position, of course. There's nothing romantic about blowjobs.
Feel free to write plenty of non-consensual sex before that point, though; everyone knows that being sexually abused makes a boy soft and cuddly, not hard and bitter. Think of rape as meat tenderizer for ukes.
Reply individually to every review you get. Don't worry about whether it interrupts the flow of the story; it's far more important to make sure your reviewers know you love and value them. Otherwise they might stop making that review number go up and up, and that number is your score.
This is the whole point. Why else would you write? To practice the art and craft of storytelling? That's silly. You need to be the one with the most cake. To increase your review count, you can sometimes entice chatty people to hold entire conversations in the reviews.
Even better than reviews. Be sure to showcase it in your profile.
Ultra win! Pimp it like it's your baby sister. The best fanfic of all is crossover fanfic, because then you can tap the other writer's attention-pool, increasing both your scores. One caution: in the rare case that someone writes a thoughtful or challenging exploration of your characters or setting, be sure your appreciation is lukewarm and generic. You don't want to encourage that kind of thing!
This is your ultimate goal. This is what it's all for. The Olympic gold medal of online fic. When people shift their attention from your story to you, when they start talking about what you say and what you like, you have won the game.
You can keep this stage going as long as you like, even if you don't write much more than the occasional teaser. Nice as it sounds to keep it up forever, though, for some reason the winners of this game seem to lose interest in their victory around the time they achieve gainful meatworld employment. Scientists theorize that toxic 'paycheck radiation' is to blame.
-=Appendix 1: How To Write Slash Like a Little Boy=-
Because little boys are simple creatures, this section can be presented in the form of a bullet list.
* Main character is a badass who kills people.
* Sex happens. Whenever, wherever, whoever. Make sure it's kind of awkward and gross, because that little dash of realism makes things happen in your pants.
* Plot is when people die or buildings burn down. Cars are sub-plots.
* Describe weapons, dead bodies, and sex. For extra credit, describe ninjutsu. Cars don't need descriptions; just give the make and model.
* Keep the love interest mostly off camera. Making him the enemy is a handy way to do this.
* When you're ready to stop, kill either the love interest or the narrator.
- Combo bonus: the narrator kills the love interest.
- Double combo bonus: the love interest kills the narrator, preferably mid-sentence.
- Triple combo bonus: Narrator kills love interest, then dies in a way that renders all his previous actions futile. Careful, though! Guys going for the triple combo can end up writing literature, which ends the game.
* The payoff is the big bloody finish, so keep your stories short and nasty for maximum kill count.
* Ignore reviews. If you cared what girls think, you wouldn't be writing about assfucking in the first place.