How do I wire a 220v circuit to accept a 110v plug and play spa?

If the circuit is wired up with a 3 conductor cable, 12-3 etc. you could just instal an ordinary duplex outlet in a box connecting either the red or black to the brass terminals, the white neutral to the nickel plated, and the ground wire to the green screw. If the circuit was wired up with a 2 conductor cable, you don't have a neutral and it won't work.

The way Labman described connecting the wiring is correct, just remember that any time you use the white wire for a neutral you need a forth wire to use as a positive ground.