I'm getting into the groove of writing book 2 now after some slow progress to start off with. I've found a soundtrack that's helping me write - a compilation of Johnny Marr tracks. I don't normally like lyrics with my writing music but having the same songs on loop eventually becomes like a sonic wallpaper more than anything else. (Book 1 was written initially to U2's Joshua Tree album in case anyone's interested).
My process has improved over the years. I start out with notebook, jotting down random notes and then plotting key points out over 5 acts.
Then I flesh things out into much more detail. The plans for book 2 are something like 15 pages long now but again only cover the key events. I still allow the characters to lead me in the writing.
I also sketch out characters to help me 'see' them in better detail. Here's some Chosen and their boss....
We've still not settled on a title for Book 1 which is, I must admit, driving me a bit mad. It's like having a child in your life who hasn't got a name... hopefully soon though.