Planning is always important, and planning effectively comes with practice and experience. By making games for jams, or just when you have the time, you'll get a feel for how long it is likely to take to do different parts of the process. It doesn't mean you'll always get the balance right but you'll be able to succeed more than fail in planning.
For this jam I overscoped a bit, but by knowing how long parts of the process were taking and having a rough idea of how long other stages might take, I was able to rescope and deliver something playable that had the core pieces of my original scope.
