1. Make sure you have gpg and the keybase cli installed.
2. Import your Keybase private/public key pair into gpg.
3. Add your git email address to the key, push the changes to keybase and configure git and Github to use your key.

Besides commits, you can also sign annotated tags with the -s option (e.g. git tag -a v1.0.0 -s -m "I am a signed tag!").