how do I compress the brake piston so I can slide the caliper over new pads?

open your break fluid resovior get a big enough clamp and use your old break pad put the pad adgainst the pistion and take the clamp and put the top part of the clamp behind the caliper and the bottom part of the clamp adgainst the old pad and start closing the clamp until ur pistion is all the way closed install new break pads

special tool turn or twist back inward,(don'tuse C-clamp)you will damage something.