Do you know how you roleplay? Yeah, just like that, but add in erotic elements into it.
If your talking just straight up the aspect of roleplaying in general, then things are much more complicated. The best way to look at it is if you are writing a story or book, but rather then you are the only one involved you are doing so with a partner. Being a group effort, you want to make sure you are controlling the elements of your character while allowing the other player control the portions related to their own. There might be some overlap, but the important part is to view it as a team effort in which you should be working together rather then against each other for control over the narrative.
As others say though, looking into what others write can help and you should likely start off with something more simple to get into it. It might involved a back and forth between you and another person where you give a sentence or two in response. As you get more into it you can start adding more detail into it and finding a certain flow with your responses. The biggest thing is really finding a partner and communicating between each other and working out what working the best for both of you. Everyone has their own preferences and so certain things might work fine for one person but they might not for another (a good example is some people who prefer using 1st person instead of 3rd person for writing).