When you get the stock deck out you will see that the brackets on the side can be removed with some philips screws. Your new deck should have a lot of pre drilled holes on the side of it, oh and remove the mounting "cage " that comes with you new deck, you wont need to use it. line up 2 of the holes in the radio with the 2 holes in the the bracket, make sure to shorten the screws so you don't pierce anything inside the radio. It is a little tricky to get the right depth, it took me a few times of screwing the bracket to the new radio, then screwing it back in the dash, putting the trim back on and so on.
Once the depth of the radio is figured out, you can mount the pocket. I had the radio out of the car with the stock brackets screwed in place on the side. then I just set the dash trim piece around the radio to figure out the depth of the pocket, then I marked and drilled over-sized holes in the bracket for the screws that hold the pocket to the bracket. I made them oversize so that I could move the pocket in and out to get it to line up perfectly around the dash trim piece.
Dom Tuned 2003 WRX Wagon
If it's worth doing, it's worth overdoing