If you want to have something "finished" (at least playable and game-like) for the jam then I would say keep it small to start. Some things to keep in mind:
- Focus on the core gameplay loop to start, worry about the menus and other extras later
- Use git/version control from the start, it's super easy to setup, and ensures you won't lose your work. Commit after every change you make, or at least daily.
- Try to get a super simple MVP done first, then add more if you have time
- Once you have something that just works, upload and submit that, even if you have lots of time left. This way, you have everything setup on itch, and have worked out any kinks with the build process, so you're not scrambling last minute to figure out why something isn't exporting properly or isn't working in the web player.
- Upload new version every day or after every decent change you make
- Depending on what experience you have other than dev, don't spend too much time on things like art or sound, especially early on. Make use of free assets or just use simple shapes to start (then you can replace them later if you have time)
- If you like the idea and want to continue working on it, you're always free to do so post-jam (I often do a bit to at least add a bit of polish, and things like menus etc.)