1. Click on 'Submit your project' button.
2. Upload a new project if you have not uploaded the game on itch.io before
3. Fill up the page with all the necessary details.
4. Save the page and make your project public. Save again.
5. Come back to jam page. And submit that project here from your dashboard after filling out the new details required.
Basically, you first upload a game on itch.io and then upload it as an entry to the jam.