Turn off power at the breaker
Verify power is off with a tester
Identify the line, load, and traveler wires
Connect the ground wires together and to the switch ground screw
Connect the incoming hot wire to the common terminal on the first 3-way switch
Connect the switched hot wire to the common terminal on the second 3-way switch
Connect the two traveler wires between the traveler terminals on both switches
Connect the neutral wires together in the box if present
Secure all wire connections with wire nuts
Mount the switches in the boxes
Install the switch plates
Restore power at the breaker
Test the switches from both locations