HOST PLAYER ONLY STEPS:
Look for your friend's summon sign on the ground.
Press the confirm button on your friend's sign and scroll over to ok and press the confirm button again

PHANTOM ONLY STEPS:
Place your sign on the ground and wait for your friend to summon it

Note: If you cannot see your friend's sign that could be for one of two reasons,
1. It's a server issue, restart both games and try again.
2. No multiplayer password is enabled. Go to the network settings (it looks like a knight emerging out of the ground) and set one. Both players need this