Usually, I write down all these ideas and then come up with a basic framework (like for instance, I come up with the main characters, the climax of the story, how I’m going to build up to it and how it’s going to end) and then make up the details as I go.
Basically just write anything you have down (Even if it’s absurd) 
Because sometimes ideas for other stories may work well for the story you’re currently working on