You have to make a game on the Jam's time frame, after (or before) that you have to set up an itch.io project on you dashboard to be able to submit your entry using the "Submit your project" button (your game has to be published and public first). After the submission period ends the rating period starts and everyone can play each other games and rate them based on the jam's categories. And for the cash prizes (if you're interseted in that) you'll have to read what each jam says.