Turn off the power at the breaker
Verify the circuit is de-energized with a voltage tester
Remove the switch cover plates
Remove the existing switches from the boxes
Identify the common terminal on each three-way switch
Connect the incoming hot wire to the common terminal on the first switch
Connect the wire going to the light fixture to the common terminal on the second switch
Connect the two traveler wires between the traveler terminals on both switches
Connect the neutral wires together if present in the box
Connect the ground wires to the green ground screw and bond to the box if required
Reinstall the switches into the electrical boxes
Reattach the cover plates
Restore power at the breaker
Test the switches from both locations