I agree with what people said...

First point is: just write. Write something. Write a draft for a scene you know will happen. Edit a scene you know how to improve. Write a line out for a part or a character. If you have your story planned out, you don't need to go in order. Just write your favourite first.

Second: do it daily. It doesn't matter how much, but like an hour or 500 words or so. Every day unless there is a really good reason not to. It need not be good, it need not t be final. It must be written.

Third: write however. Find what suits you. When, where, how. E.g. I can hardly write on something else than my computer. Maybe pen but only for notes. But that is totally personal.