Clone your version on your account down to your machine from your account, e.g,. git clone https://github.com/<yourgithubusername>/brranching.git
Make sure to track progress upstream (i.e., on our version of brranching at ropensci/brranching) by doing git remote add upstream https://github.com/ropensci/brranching.git. Before making changes make sure to pull changes in from upstream by doing either git fetch upstream then merge later or git pull upstream to fetch and merge in one step
Make your changes (bonus points for making changes on a new feature branch)
Push up to your account
Submit a pull request to home base at ropensci/brranching
