Kisses can be a good bit of trouble. There can be very much or very little going on. A large mistake people tend to make is focusing solely on the action of the mouths and nothing else. You don’t want to write on only tongues and lips or, as a friend of mine put it, it can come across as a big slobber fest.
Body language is super key in even the smallest of kisses. Always remember that.
Breaking it down into the other senses can help as a jumping off point, though.
You have touch. There’s clothes (more than likely) or at least skin. What’s under their fingers could be smooth, could be hairy or bristled. It could be worn through cotton or silk. Hair is there to card a hand through. Bodies press against bodies and you get warmth and rasping clothes/skin/hair. People have moles, have scars, have shivers in their legs or rigidness in their muscle.
Not every character has baby soft lips, either ; Some wear too much gloss, or waxy lipsticks, some have chapped and scabbed lips. Consider the conditions they’re in, too. If it’s raining, things are slick and tack together, especially if there’s sweat. Winter and fall can bring some nasty wind burns or cracks in the skin.
And with mouths and touching, that’s where a lot of writers stop. But that only covers touch, and possibly taste.
There’s sight. Most people have their eyes shut during kisses, but if there are kisses to other bits of the body, it can be explored. Either way, eye contact, especially in the midst of a kiss, can communicate quite a bit. Why does your character have their eyes open? Are they shocked? Are they struggling away? Eye contact isn’t always negative, either. It can be reassuring, or teasing. And then you get people with eyelashes long enough that if they flutter them it can actually tickle, which isn’t so much sight but it has to do with eyes.
If your character is devoid of sight, it’s a great opportunity to play on other senses. Hearing, for example, would be great to play upon. One would hear the light pops and slicks and puckers of lips and tongues, but there’s the rasping of skin too, and of clothes. Maybe they’re on a leather couch. Leather creaks and makes odd little farting sort of noises. Or if they’re on the ground, the dirt scratches under them and the grass rustles. You get the idea. There’s breaths, moans, whimpers, the loud, cusped sounds when things run over your ears.
Scent plays a more minimal role depending on the location. Or maybe one character has rancid breath. Or particularly sweet smelling breath. It’s a more limited sense.
Taste is the one most young fanfic writers can go overboard with. Always remember that skin (and yes that includes the male bits) tastes like skin. It’s salty and bland and sticks on your palette, often, longer than you’d like. Excrements are what hold the taste and, gosh, that sounds so clinical but you know what I mean. Cum has taste, it’s usually salty and bitter. Sometimes it’s a little sweeter and fruitier, depending on diet, but overall, it’s salty. Same goes for the female…discharge? I guess you would call it. Lip glosses aren’t always flavored. Most lipstick just tastes like oil and wax. Again : Skin tastes like skin.
When food is incorporated, things that are too sticky can make a mess. Things like chocolate can become hard and monotonous to lick off.
The most important thing about kissing scenes though, is that they are never always perfect. People’s noses will bump, their chins might knock, their teeth could click (which hurts more than you’d think), a whole mess of things could happen. Some people don’t know how to kiss. Their lips move like they’re a fish, or they lose their breath. It’s important to show these things because it develops characterization.
It’s also worth mentioning here that spit dries rather fast and isn’t suitable, at all, as lube. Nor is pre-cum, really.
Never be so desperate for something new that you fall back on the cliche of writing “…He/she did that thing with their tongue/hips that drove me wild.” It tells your reader nothing and it’s a terrible place holder.
I think the thing with kissing, and almost anything to do with writing really, is to work in moderation. Find a good balance. You never need to have all these things, and sometimes you need none of them Sometimes saying “And she kissed the line of her brow” is quite enough. No kiss should be perfectly identical to another. Unless the intension is monotony.
In a nut shell :
Body language is key
Never will it always be perfect
Make kisses unique
Utilize different senses