People who write erotica, how do you go about it?

I read quite a lot of erotica through sites like Literotica. I have ideas for my own stories and would like to post them but before I start I would like to know how you go about it. Do you plan the whole story before writing or do you just start writing with an idea and and add the details as you go along? Ive never written a story or anything like that before so I guess im just looking for tips. Thanks