I've done this many times but every time I attempt to do it anew many months have passed and I always have to look it up again. Here is a quick reference for myself and others to do this quickly and easily.
First create the ssh key on your local computer and copy to your clipboard:
~$ cd ~/.ssh && ssh-keygen
~$ cat id_rsa.pub | pbcopy
Then in GitHub go to "Settings > SSH Keys > Add SSH Key," paste in the key and name is something appropriate so you can keep track of your keys.
~$ git config --global user.name 'My name'
~$ git config --global user.email myemail@host.co.uk