I'm starting work on the third book in the fantasy trilogy I have been writing. It has been quite a long process, with the first book published in 2012, so step one, for me, is to reread the two books from the trilogy that are already out there.
It's an interesting task. As I read, I'm reminded of details that I made up, more or less spontaneously, at the time, and then promptly forgot.
For example, there is a little scene where Willowtide, the young elf woman who is the hero of the book, buys an amulet just before she sets off on her first adventure. I had completely forgotten about this, but now I am very tempted to somehow weave this forgotten detail back into the ongoing story.