I don't realy keep a writing journal but I think it's a good idea to know how to keep one.
It's really easy if you ask me. You can keep anything you want in there, whether it's for your diary to keeping your character's thoughts throughout the story. You always need to know more than you tell or show your reader because then if you would like to add something here and there then you can without having to go through with scratch writing.
Keeping your character's interviews is important. Say you have a villan on the outside, but you have an interveiw with him (in your head but if you would really like to have an actual conversation go on ahead) and you find out that you want to make him nice and kind on the inside. Then oh no, you're halfway through your story when you forget about making him nice. Now if you forget that important piece of information that may be the twisting tale to you story then your S-O-L. Really though if you would've taken your time to keep a journal for crying out loud maybe this wouldn't of happen.
To keep one it's simple, simply keep your's or your character's thought in there so that if you feel you have a good enough idea you can follow through with it.
Thanks for reading