There are different type of lemons to write. And it differs if your write them from a male or female point of view. And character of course.
There are the smut versions and more romantic versions. The smut are the ones that really don't have a plot. There are also the darker kind of fics. That sometimes includes rape. The language can differ too. If you refer to genitalia for example there a lot of words to use. And it depens on the story you wanna write.
A story that has a more darker theme will use more harsher words. A more romantic story will use the more romantic version words when one refers to genitalia or other body parts.
The best place to start is to read lemon stories. It will give you an idea.
I write lemons. Feel free to check my stories out. The continuation that is posted here will have a lemon in a later chapter, but I have other Inuyasha stories on FF.Net (Angelhart-79) that are mostly all lemon one shots.
My first lemon was Tree Level.
(story would be rated MA on this site)
The most important thing is that you should never be ashamed to write a lemon. You can make them as graphic as you like. Things will get easier as you write more.
Try to get a feel for it when you write the story. It is easier to write one with a plot to start with. When you get to the scene where the 'magic' happens
and you don't know where to be begin, just wait for an hour or so and read your story from the beginning. As a reader you will find that once your in the story and the mood is set your mind will aid you along, because as reader you will expect things to happen. And sometimes your imagination runs ahead. The only thing you will have to do is write it down and take it from there.
If you have any specific questions don't hesitate to ask. I don't mind to help a fellow author
Good luck!