This section is very much incomplete. Currently, take a look at the test suite and the code to gain insight into how Gittern works.
The low level interface is where the magic happens. This is also where you’ll absolutely need to be familiar with the git model of blobs, trees, commits, and how the fit together. Seriously. If you don’t e.g. know what a tree is (in the context of git, of course), probably won’t understand how to use this, and theoretically you might be able to break your repos when using the low level interface. Proceed with caution.
If you want to learn how git works, there’s plenty of resources. I’d suggest the following, in order: