Yo, 

Miro is an excellent Product management

When you start a game jam have a pre-production meeting where you sit down and decide what game you are going to make. 

Also depending on how much time you have create a GDD ( Game design Document) this essentially outlines everything that will be included in the game from story to gameplay I have a simplified template I can send to you if you want.

Personally I have a bad habit of starting something grand so if you are anything like me, shrink the idea/project to its smallest form and build from there. 

I hope this helps. I am also thinking of participating in the jam if my PC starts working again.