Lately I've just been prying between the outside hooks of the caliper and the rotor to compress the piston. It is easiest to compress it first, before taking anything apart. You can also use a clamp with everything assembled, from the back of the caliper bore to the outside pad. The brake fluid level will rise in the reservoir on the master cylinder. If you've been keeping it full to the top, make sure it doesn't overflow.

Also as you're changing the rotor, you will need to remove the caliper bracket from the knuckle (2 big bolts on the inside) to have clearance to remove the rotor. Remove those bolts and take off the whole assembly: bracket, caliper, and pads all still assembled. Then once it is off, you can pull the old pads out and put in new ones (of course, never run old pads on a new rotor).

Thoroughly clean the friction surface of the new rotor with brake cleaner before installing it. They often have a grease or other coating to prevent rusting during storage.

 

damn those rotor retaining screws! i had to use a hammer and a punch and punch them out a quarter turn at a time.

 

why can't i reuse old pads?
i'm buying new ones for the rear, but the ones in the front I recently replaced and they're almost brand new

 

They've worn to match the grooves in your old rotor. Using them like that might put grooves in your new rotor. Some people, if they're really really cheap, try to sand them down flat again.

 

alright, well i ended up using the old front pads, as they're only about 2 months old and i really don't want to buy new front pads again

i used new pads on the rear though

I ended up having to drill out all of the rotor retaining screws, and the rear passenger rotor piston was the biggest PITA in the world
took me an hour to compress that piston

anyway, my brakes now work great, not to mention look 10x better