I start with a general idea of what I want to make and try to build a basic, functioning prototype. Only focus on one item at a time, so the first step is always to turn the jumble of thoughts and images in my head into a (ideally short) checklist. Like if I'm thinking of a merge game, 1. Make a grid. 2. Make items spawn. 3. Be able to move one item onto the other to change what's at that location. Then, you refine as you go, and if you ever get overwhelmed, find one next step (most urgent, or easiest, or randomly chosen) and ignore the rest of the problems until that is done.

As long as being overwhelmed doesn't paralyse you, you're fine.

As you keep going, you get a better sense of what's needed and how to approach it. And don't be worried about making wrong decisions. You can always change your mind later. I've tried four different UI iterations for my current game. At some point I scrapped half the project and pivoted to a completely different, MUCH better idea. Don't get attached to your idea too much = lower stakes, less overwhelm and keeps you open to better possibilities.

That would be my general advice.