I’ve been meaning to post this somewhere useful for a while now, so here it is.
Machine A wants to connect with SSH without passwords (with cert) to Machine B
On Machine A:
Make sure you pick no pass phrase.
scp .ssh/id_rsa.pub user@machineb:.ssh/id_rsa.tmp
On Machine B:
cat id_rsa.tmp >> authorized_keys chmod 600 authorized_keys
Connect as normal from machine A to machine B