It's a good idea to upload and test early versions of your game, as there may be bugs that don't occur when testing in the editor, only after uploading and playing in a browser.  Follow these instructions to (hopefully!) successfully build your Unity game for submission to the jam.

In Unity Editor:
File -> Build Profiles
If Web is not the active platform, select it and click Switch Platform (you may need to install the Unity Web Module)
Open the Scene List and ensure all required scenes are checked, with the scene that should open on launch at the top of the list
Open Player Settings (top of Build Profiles window)
Under Resolution and Presentation, set Default Canvas Width and Height (1280 x 720 usually works well)
Under Publishing Settings, set Compression Format to Disabled
Click Build in the Build Profiles window and create a build folder

Once the build has completed, zip everything in the build folder (Build, Template Data, index.html)

On your itch.io dashboard, create a new project
Set Kind of Project to HTML
Upload your zip file, and check "This file will be played in the browser"
Set Embed Options to Auto Detect Size (don't enable itch.io's fullscreen button as Unity adds its own)
Save and view page to check the game plays as intended.

If you're happy with your game, set the project's visibility to public and submit it on the CSC3231 Game Jam page!

You can replace your uploaded file with a new version any time before the jam ends (after this the file becomes locked).

Try and get your game uploaded and submitted at least a few hours before the jam ends to ensure you don't miss the deadline and have a bit of time left to iron out any last minute bugs.