It is very easy. The six big plugs at the bottom have an inner portion with an outer ring. The ring has a gap on each side. Just slide a flatblade screwdriver through the gap and under the center plug and pry it out gently. The screw type plug at the end just needs to be scewed out with a small phillips head. Mine didn't come out that way so I gently pried out the center with a small flat screwdriver. Keep the plugs if you are installing new Subaru sides since they don't come with new plugs.
I just picked up my painted side skirts today! Looks really good. Remember when you give the body shop your skirts, take off ALL the clips. They can be lost easily. Just keep then in a baggy and put them in your glove box. By the way, how much you paying? I payed $100.
If they did everything correctly, $100 sounds like a good deal. I bought the OEM painted sides (silver) for $240 from the local Subaru dealer. I had already taken my car through carwashes where wax was applied so I decided that getting them clean enough to paint was too much of a hassle. If the cleaning step is not done properly the paint will crack off in time. Also this is a somewhat flexible part so I would recommend adding a flex-additive to the paint. Not all shops do this automatically since it costs a little extra to do.
I decided not to do it this way. I've taken my car to coin-ops, so I can't be sure that there's any silicone residue from the soap or wax. Plus, I've put on tire shine stuff a few times and it may have sprayed on the skirts. Thinking about the actual difference it will make on a black car, I'll just hold off for now. I'd rather apply that $100 to an up-pipe, mbc/ebc, intake, or exhaust.