The wires going to the throttle body switches in the ECU wiring harness should be Blue/Black (WOT switch), Red/Blue (Idle Switch), and Brown/Blue (Ground).
You cut the WOT switch wire and join the loom side to your normally-open pressure switch. Insulate the connector side, you don't need it. The other side of your pressure switch needs to be grounded - use the Brown/Blue ground wire to do this, but don't cut it as you still need it for your idle switch to work! Leave the connector plugged in to the TB as normal.
The above assumes you're wiring up the pressure switch outside of the throttle body, directly into the wiring harness. If you're doing it inside the TB the wire colours may be different - but you should be able to match up what does what from the above.