1. Create a folder named “submission”
…
4. Zip the submission folder so that it’s named “submission.zip”
One common gotcha is that right-clicking on the submission/ folder and using your OS’s Compress "submission" (on macOS) or Compressed (zipped) folder (on Windows) will typically create an archive where the folder itself is at the archive root (so you end up with ./submission/main.py rather than ./main.py like @kwetstone described). Instead, you should be inside the submission/ folder, select all of the files you want to include in your submission, and then use the right-click command.
Or, the easiest way to do this correctly is to not use the right-click ZIP creation functionality and instead use make pack-submission like @kwetstone suggests.