1. Place it in the right place

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

2. change permissions on file

sudo chmod 600 ~/.ssh/id_rsa
sudo chmod 644 ~/.ssh/id_rsa.pub

3. start the ssh-agent in the background

eval $(ssh-agent -s)

NOTE: If your shell is fish, then just execute the ssh-agent -s and then export those printed variables.

4. Make ssh-agent to actually use the copied key

ssh-add ~/.ssh/id_rsa

No need to import public key.