When you add new scenes to a LevelList ScriptableObject, it should automatically update the build settings to include it. However, to double check go to File->Build Settings... or press Ctrl+Shift+B to confirm. Most of the build details are already done, but in case you want to change things you can modify them in Player Settings... in the Build Settings window. Our Gunfish Rectangle is windows, so make sure you're building on Windows.
Once you're ready to build, in the Build Settings hit Build or Build and Run. The first time you do this, it will ask you for a folder to build to. You can select anywhere but keep in mind that the folder you select will be the root, so if you're doing multiple builds make sure it's in a subfolder. I recommend putting it in the Unity Project root (Gunfish/Gunfish) under a newly created Build folder, which is already set up to be ignored in Git.
Once the build is done and you've verified it works on your own machine, right click and compress to a Zip. Then, create a new Git release. For Choose a tag, increment the version number (i.e. for v0.1.0-beta change it to v0.1.1-beta). For viable MAGFest builds, start labeling them as v1.x.x and no beta. Target should be main if it's up to date, otherwise develop. Release title can just be Gunfish, and you can choose whatever you want for the description. I recommend Generate release notes which makes a description automatically, and it's honestly not bad. In the Attach binaries by dropping them here or selecting them. section, drag and drop the Zip from File Explorer. Now, you can Publish release.
On the Gunfish Rectangle, navigate to the Gunfish tags. Permissions should already be set up to Ryan Kann, but in case they aren't log into your own GitHub and you should have permission. Click on the release you just made, go down to Assets, and download the binary Zip for the Windows build. You can add it anywhere, though I recommend the Desktop so its easy to find if needed by MAGFest staff. Finally, right click and unzip it. The executable will be inside this folder.
Here are detailed instructions from Microsoft to Add an app to run automatically at startup. To summarize, Windows logo key + R, type shell:startup and press enter. Then, add a shortcut to the Gunfish .exe file in the build folder.