I'm learning basics of Git, but I can't grasp one concept. I went through major tutorials on YouTube, Github, Attlasian, Gitguys and Git-SCM. Unfortunately those tutorials are written by programmers who aren't teachers.

**Here is the thing:** 

I want to contribute to an existing repo (unit testing).
But I'm only allowed to do so in a separate branch that I also have to create.

What Git commands do I need to download this project, add my code, create a separate branch, upload project with my code? 

PS. For your convenience - 

* name of the new branch "feature/unit-tests-from-z"
* link to the repo "<https://link.git>"
* name of the current branch "mainBranch"