How do you set up a towing wiring harness for a 1994 suburban which includes a towing package?

Have you looked under the vehicle? I'd be surprised if the wiring harness wasn't already there and taped up under the vehicle.

Barring that, most business that sell and install trailer hitches will also install trailer wiring harnesses.

If you're a DIY, go to the local auto parts store and ask. They probably have the wiring harness you need all setup to plug into the manufacturer's wiring harness.