Open TODO file, if there is one, and see if there is anywhere you can help. Otherwise, look at the issues.
If you want to contribute, fork https://github.com/keithjjones/7zip-cpp and perform your modifications on the "devel" branch.
If you send a pull request we will review the changes and merge them appropriately.
The "master" branch is always the working code. The "devel" branch is working code, but it hasn't been accepted into "master" for the entire project by @getnamo. Any other branch consisting of new features typically starts with "devel-" and contains the feature's name. It may or may not be working depending on the contributor. These branches are branched from the "devel" branch.