Before opening your first pull request to Yoga you have to know how to get the code, install build time dependencies, and test the code locally.
$> git clone https://github.com/facebook/yoga.git $> cd yoga
$> git submodule init $> git submodule update $> brew install buck
$> buck build //:yoga $> buck test //:yoga
Now all you need to do is make your change and test it before submitting a pull request for review. Below is the general structure of the repo and where you may want to make your change. One you have made your change see the testing documentation for more on how to test your change.