A good way to practice is also doing some mini jams yourself. For this jam in specific the theme will be based on one of GMTK's videos, for for some practice you could put a list of some of those videos into a random selector and let it choose one at random and try to work on it for the next 3-4 days or so, giving yourself some time. Maybe even a week when starting out. Noting down what you had trouble with so that you can focus on learning that aspect more. Just don't pressure yourself, the point is to have fun not to stress out. But this can give you a theme from which you come up with an idea and try to work on it. Most people's first jam doesn't turn out that great, so don't feel to bad if you don't get it to where you want it to be. From what I've seen many people don't even end up finishing. So there really isn't that much pressure.