you mean to say that you want syntax or any troubleshooting assistance while doing it.

anyway following is the command. first of all launch vm with your choice of image which you done already. add rules to it. like,

nova secgroup-add-rule default icmp -1 -1 0.0.0.0/0
nova secgroup-add-rule default tcp 22 22 0.0.0.0/0

for cirros instance.

ssh cirros@INSTANCE_FIXED_IP/INSTANCE_FLOATING_IP

for ubuntu instance..

1. nova keypair-add OSKEY > OSKEY.priv (where OSKEY is the one you generated from ssh-keygen command)
2. chmod 600 OSKEY
3. now launch the instance with the OSKEY as the key
4. ssh -i OSKEY.priv ubuntu@floatingIP

hope that helps