Just bleed the air out of the circuits you opened up. Crack the cap on the reservoir to let the fluid run down freely. When air bubbles stop coming out of a bleeder screw, tighten that one and wait for the other one to flow clear fluid, then close it too. VERY IMPORTANT: NEVER push the brake pedal more than half way to the floor to work the new pistons out of the calipers to get the pads to hit the rotors. Doing so can damage an older master cylinder. REALLY REALLY IMPORTANT on GM cars: Pressing the pedal more than half way will trip a valve in the master cylinder when one caliper piston comes all the way out first and that side starts to build pressure when the other side hasn't gotten that far yet. At that point brake fluid will not flow to the second caliper. We'll discuss how to solve that if it becomes necessary.
If you didn't let the reservoir run empty, this gravity-bleeding works well and you don't need a helper. If the reservoir was allowed to run empty, you may need to irritate the brake pedal a few times to get the flow started.