$ apm develop <package-name>
$ atom -d

apm develop will clone the repository on ~/github/<package_name> or in the directory defined by the variable $ATOM_REPOS_HOME. It will also install dependencies and create a link from ~/.atom/dev/packages/<package_name> to the repository directory.

atom -d opens atom in development mode with the cloned package installed. Since $ apm develop creates a link to the cloned repository opening Atom in development mode will automatically load the package. Note that the cloned package won’t be loaded outside development mode.

If working on a PR go to the repository directory and update origin to point to your fork:

git remote set-url origin <https://github.com/USERNAME/REPOSITORY.git>
git remote add upstream <original-repo.git>
git fetch upstream