A 2015 impreza is going to either be a CVT or a manual, assuming you're in the US anyway.

Changing the CVT fluid first involves highly specialized fluid. You'll need Subaru CVTF-II for yours. A drain and fill will take somewhere between 4 and 7 qt depending on how long you let it drain. Draining the fluid out is easy; filling it back up, not so much. There is a fill plug on the back-left of the transmission. You need to fill the fluid with the car running in park and the transmission temperature between 95 and 135 deg. F. Also, because it's a vertical plug, you'll need some way to get the fluid into the hole (it's like filling a rear differential fluid) so without some flavor of pump it's going to be very difficult.

how the hell do you get to your oil pan?

Jack and jack stands, or Rhino ramps. I use one of these wrenches at work in my oil cart, the angle is just enough to get to the drain plug through the undershield.

I busted out that plastic cover for where my fog lights would go if I had them. I looked online and I can just buy actual fog lights for $50 on Amazon instead of replacing the cover. Anyone know how hard that install would be?

Personally i would avoid the generic add-ons from amazon and stick to the subaru accessories kit. Black interior or beige interior. These come with everything you need, including the lights, trim panels, relay, switch, and mounting hardware.