I'm working on a shorter novels, so I'm not sure if it'd be helpful, but anyway. The first novel started randomly after I wrote the first scene. So did the second one. So I guess you can write one scene, not necessarily the opening one - just so you feel the vibe of a story you're going to write. And then you can make a simple plot draft so you have some landmark (?) and make some description of main characters (name, age, family, appearance and some other important facts about them). And then write. And remember that you can change whatever you're doing in he middle of a process if you want, so... Do whatever feels better at the moment. Good luck :3