So you want to write code and get it landed in the official OMRChecker repository? Then first fork our repository into your own Github account, and create a local clone of it as described in the installation instructions. The latter will be used to get new features implemented or bugs fixed.
Once done and you have the code locally on the disk, you can get started. We advise to not work directly on the master branch, but to create a separate branch for each issue you are working on. That way you can easily switch between different work, and you can update each one for latest changes on upstream master individually.
For writing the code just follow the Pep8 Python style guide, If there is something unclear of the style, just look at existing code which might help you to understand it better.
Also, try to use commits with conventional messages.