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