Again, each car is individual so providing the step by step wiring will apply to only a very select few cars.
The basic procedure you are going to use is as follows:
Locate the non-engine related wires existing on the car and seperate them from the engine related wires. Locate those same wires on the WRX harness. Now you "simply" add in the WRX engine related wires to the existing non-engine related harness and voila. Then you have all your CEL's pulled and spend the next two months retracing everything trying to find where that missing ground is or those two crossed wires.
Unfortunately, I have not done this yet but I am looking to do it in July. Yet unfortunately again, it will be of little help to you since I will be putting this into a 96 L wagon, not an RS.