1. Edit the file containing the public SSH Key (for example c:\users\<username>\.ssh\id_rsa.pub) and copy the entire contents to the clipboard. To check that you have the correct file, the contents starts with 'ssh-rsa', 'ssh-ed25519', 'ecdsa-sha2-nistp256', 'ecdsa-sha2-nistp384', or 'ecdsa-sha2-nistp521'.
2. Go to your GitHub repository Settings/Deploy Keys and click on Add Deploy Key:
3. If you get the error "deploy key already in use" you will need to generate a new SSH Key (or use a GitHub team in a corporate account).
4. Give the Deploy Key an appropriate title and paste the public key into the Key field.