Add existing key to SSH

cp /path/to/my/key/id_rsa ~/.ssh/id_rsa
cp /path/to/my/key/id_rsa.pub ~/.ssh/id_rsa.pub

Change permissions on file

sudo chmod 600 ~/.ssh/id_rsa
sudo chmod 600 ~/.ssh/id_rsa.pub

Start the ssh-agent in the background

eval $(ssh-agent -s)

Make ssh agent to actually use copied key

ssh-add ~/.ssh/id_rsa