When the jam starts the theme will be revealed, and you have 9 days to make a game using that theme as inspiration. You can submit your game to jam using the jam page on itch any time during those 9 days. After the jam is over, there will be a week long voting period for people to give feedback :)
Like Axel said, once the jam officially starts, you can submit your game through the jam page. A button will appear on the page for you to do so . You will need to upload your game to itchio first as a project and then publish its page and make sure that it's public. When you click the submit button, there will be some questions you will need to answer about your game and then you select which one of your projects you want to submit for the jam.
