Here's a breakdown of how I prioritized work. Maybe it'll help you.
Make a few idea's, decide on the one that I'd enjoy playing after a short time programming and enjoy all the way through.
Create a proof of concept. Make the minimum viable game that is playable and enjoyable. Simple shapes moving around, placeholder images if they are needed for things like game and background. Focus on making your player movable, or giving the player the ability to take the action they take. Get basic collision working if you can with basic objects in your environment if those exist.
Publish. Make sure you can get it working in Itch.io. Not all features work the same in an editor as they do in a webapp.
Add a win or lose condition. You fall into the pit as Mario. You destroy all the balls in puzzle bobble. Use your basic interaction and make it do a basic task. Pop up a message when you win or lose.
Add the ability to restart the game. Create a play again button on the win and lose screen.
Add enemies, ai, etc. Make basic elements that interact with and oppose your player.
Add a start screen. Tell the player what to do, how to win, and how serious the game is, etc.
Add basic placeholder sounds. Consider open source effects to start. Make sounds when things bump, when you kill the enemy. A victory jingle when you win. A sad sound when you lose.
Make music. Make it loop cleanly. Make it easy to listen to for hours, because you will. Add it to the game.
Increase scope. Make full levels. Add more enemy types. Add more weapons. Make the game more fun. What's the "aha!" moment that you want to shoot for. Try to program something that's fun to interact with beyond the proof of concept.
Improve textures, make real sound effects, clean up start and end screen. Add a pause menu if you'd like with things like volume control.
Polish. Add nice to have items. Add the extras you'd enjoy seeing. Focus on keeping the experience solid as you go.
Consider having folks test. What works? What needs help? Try to keep in mind each voice is just one opinion. Take what suggestions are appropriate and implement them in the time you have left.