Personally, when I develop a game, I try to work on the part that I’m in the mood for. When I’m relaxed and not in the mood to do work, I play my game, and write down tasks that I could do to make the game better. I’m not thinking about having the right skills/tools to finish it, just as a player what would improve the game.
Then when I’m in the mood to do some work, I go through all the tasks that I’ve written, and do the one that sounds more fun for the moment. Sometimes it’s implementing basic gameplay mechanisms, sometimes it’s drawing new assets and so on. If a task is left undone for too long, I try to investigate why, and possible change it to make it more fun for me to work on.
You don’t have to focus only on the “important” things like the gameplay. At the end of the day, game dev should be fun. If you’ve come up with a concept that you will not enjoy making, it would be better to alter it, and make it more fun to work on, even if it doesn’t look as you first envisioned it.
What I described, is my approach on game dev. Can’t guarantee this will work for everyone.