For trivial fixes (e.g. fixing typos or bugs spanning only a few lines), feel free to send a pull request with your changes. For anything bigger than that, it's best to talk to us first.
If you have a specific improvement you'd like to contribute to Shadow, the best way to get started is to file an issue or comment on an existing one. You can save wasted work this way by ensuring we have agreement on whether you're planned improvement and design for that improvement fits into the roadmap for Shadow, and won't conflict with other on-going development.
If you haven't already, it's worth going through the general documentation to get started with building and using Shadow.
When you're ready to start coding, please take a look at our Coding style and Version control guidelines.