Personally I've been pantsing (a common term for not planning in advance, based on the phrase 'flying by the seat of one's pants'), and I don't think I like it. It's too easy to get stuck, or to go on too long about things that aren't important enough to be interesting, etc. So I'm working on some ideas about how to get ideas, and how to structure them.
Here's the method I'm about to try:
Using mind-mapping software (I'm looking at Coggle probably) map everything I know about the story -- who what where etc. Then, for each node, start asking questions. Write down a bunch of answers, pushing myself to try to find multiple possibilities, because the first thing I come up with isn't necessarily going to be the best. Mark these with a "?" so I know what's more-or-less firm and what's just a possibility. Once I feel like I've got enough 'stuff' in the map, try to start making more connections between nodes.
I'll let you know how it works out!