How do I eliminate one 3 way switch and replace it with a single pole switch?
A modification of longneck's process: (not enough rep to comment)


Set the switches so the light is on.
Turn the breaker off
Remove the garage switch; use a test meter to figure out which leads the switch is connecting.  Connect those two wires permanently.  You are removing this switch from the circuit.  Cap the other wire.
Remove the other switch; use a test meter to figure out which leads are connected by this switch, hook them up to a single pole switch.  Cap off the remaining one.


To figure out where the power really flows in this circuit, you have to power this up safely, with the terminals exposed, and use a neon tester figure out which is hot, when the switch(s are) is off.