There's notches in the pistons for a special tool to engage to let you screw them in. The alternative is to remove the parking brake lever to allow you to turn the actuator screw a little, then push the piston in a little, then repeat until the piston is fully retracted. That's a lot of work and time-consuming. Most auto parts stores that rent or borrow tools will have the tool set. Harbor Freight Tools and Sears sells them too.
Once everything is reassembled, you have to work the parking brake a few times to run the pistons back out. They will not self-adjust from pumping the brake pedal like the front ones will.