I'm feeling generous today so here's a few important tips:
Be meticulous and label every wire you cut. Label it with at least the pin location number or better if you have a description of what the wire is.
Study the wiring diagrams for both cars and write down which pin location is the wire for what and try to match them up to the corresponding pin location on the new ECU.
Cut the wires so they are staggered. That way, when you clean up, there is less of a bulge in once place. Soldering and wrapping will make your wire thicker and you don't want all the thick wires in one place.
Study the differences between the wiring between your Impreza and the WRX. For example, the fuel pump relay is grounded by the ECU on the RS whereas on the WRX, it is grounded on the body. Make a note of the differences.
Make sure you look closely at the numbers of all harnesses, relays and connectors. The diagrams may have the same 4 wires listed but have them listed as 1, 2, 3, 4 and 3, 4, 2, 1.
Best thing to do it braid together the wires first and then go through and solder them all. Then wrap them in electrical tape.
Follow the diagrams closely for wire colors. Just because a wire is a certain color in the engine bay doesn't mean it'll be the same color after it enters the firewall.
I'm sure I have things I haven't covered but these are the more important stuff that remains prominant in my mind.