Pull the repositoy that you want to migrate
$> git clone <url-to-repo>
You can either create the target repository through the UI or use hub from the command line.
$> hub create -p <organization or username>/<name of repo>
Switch the origin
repository to the one you just created.
$> git remote set-url origin git@github.com:<organization or username>/<name of repo>.git
Confirm that it was switched
$> git remote -v
origin git@github.com:<organization or username>/<name of repo>.git (fetch)
origin git@github.com:<organization or username>/<name of repo>.git (push)
Push the source to master in the new repository
$> git push --set-upstream origin master
That's it!