Hi, @michael_harris. Those instructions seem to be designed for maintainers who have write access on the repo. The process to contribute to a repo without having write access is similar.
If you first create a personal fork using the Gitlab web gui, and then follow those steps cloning from your fork, you should then be able to push to your own fork and then open an MR from your fork, targeting the main repo.
Others might have links to a guide with the additional steps, but hopefully this note at least points you in the right direction.