(Mostly from quora)
Let's be direct and short with this one since I think simplicity works best to teach about said topic.
Build the tension. Show hints of attraction both ways, and have your readers begging for action.
Stay away from cliches like "our lips crashed together".
Don't go Out Of Character (OOC). If a character is reserved, don't suddenly make them go mushy-mushy.
Don't dump romantic/sexual action everywhere. The readers will get sick of it. Instead, show their character development and put the action where it's useful. Such as someone panicking in a situation, speaking nonstop gibberish, and kissing the person, the shock makes them "reset" and become interactive again.
Try to connect your characters and plot to the canon. Even if your pairing is absurd, good writing will manage to convince readers the attraction has been there all along.
Show, don't tell. Don't do mind games 24/7.
Don't go making the character(s) overly popular for no real reason. Don't make them the object of other people's affections immediately. Give characters a reason to actually be together. Like getting to know one another. Don't jump into it automatically, but let the relationship build.
Don't dedicate whole, large paragraphs entirely to clothing. Give a brief description (not a list, tie it into the narrative), and move on.
Smut; It's not a requirement for a story of any sort, so don't feel like you need to have those kinds of scenes. They're often awkward, make the character OOC, and have a completely different tone than the rest of the story.
Avoid harems, particularly with series that don't have them. Mainly as those tend to butcher the characters and make them shells of their true selves.
For any fanfiction, when writing dialogue, think to yourself; "Would this character really say this? And if so, would it be with these words and in this tone?" In short; keep characters in character.
Make the relationship natural; don't force it. Go back and read what you wrote, and if comes off as awkward or just not fitting right, go back and fix it.
Avoid flowery language. Undersell the romance and cement the relationship first.
Sex scenes should be kept to a minimum unless you are writing erotica.
If it is slash - don't automatically make one character more masculine and the other more feminine. It doesn't matter if you are writing F/F or M/M or M/F.
This is rarely how it works in real life.
There should be no mention of heaving bosoms or throbbing members. No dripping of feminine nectar or pearly beads of cum. Are you an animal? Is your character a horse or a cow? Keep it cool.
Drive the story on the emotion and devotion each character has for the other.
Never forget you have internet access. Any doubt, google it.