Once everything is painted to your liking, you can start testing and aiming your headlights. Install the bulbs into the projectors and wire up the harness. Everything is labelled, so the wiring is pretty straightforward.
Mount the projectors back into the reflector bowls, and snap the reflectors bowls back into the headlight housings. Reinsert the aiming screws and mount the lights to your car. Also reinstall the turn signal and parking light harness.
This is the time to follow standard HID aiming procedures to get your lights aimed properly. There are plenty of sources on the proper aiming of HIDs. For instance:
There is one issue regarding the wiring.
The WRX is notorious for having finicky wiring. If you do a search on "TRS 9007 wiring harness issues WRX," you will come up with 2-3 typical problems that occur on a regular basis. One of which was that the high beam solenoids wouldn't activate upon flipping the high beam switch. After some messing around, a friend and I were able to figure out the proper fix for the problem.
I turns out the main plug on the harness needed to be re-pinned like this:
If you end up having issues similar to mine, or any of the typical issues, do some reading on that particular TRS harness. There are several discussions regarding particular problems and you should be able to sort them out eventually. There is only a finite number of pin combinations on the main plug. The other fixes include reversing the plugs going to the ballasts / projector solenoids.
Once you get the wiring and the aiming right, you can start sealing up the headlights. I chose to leave off the orange bulb cover on the bezel, however there are other modding options, such as the Koji mod, for this part of the retrofit that can add a touch of personalization.
Additionally, this is where you get to choose the shrouds you will use to cover the projector housings for a clean, finished look. I chose the "ocular" shrouds offered by TRS (the retrofit source), but there are plenty of options available.
Before you reseal the lights, make sure to clean everything so you don't end up with random bits of lint inside the lights.
I bought extra sealant from the retrofit source to reseal the lights, but there was actually enough on the headlights to get a good seal. Reheat the sealant around the edges and press the lens/bezel combination back onto the housing/reflector bowl combination and tighten the (2) screws that hold the two halves together. Make sure there are no gaps and that the two halves are pressed together all the way.