Use usual steps to make contributions just like other GitHub projects. If you are not familiar with Git Branch-Rebase-Merge workflow, please read this first.
Checklist
Use same style and formatting as rest of code even if it's not your preferred one.
Change any documentation that goes with code changes.
Do not include OS specific header files or new 3rd party dependencies.
Keep your pull request small, ideally under 10 files.
Make sure you don't include large binary files.
When adding new includes, make dependency is absolutely necessary.
Rebase your branch frequently with master (once every 2-3 days is ideal).
Make sure your code would compile on Windows, Linux and OSX.