Hi and welcome to just answer.
the pistons have to be wound in a clock wise direction and at the same time pushing the piston .
you really need a wind back tool to do this.
if you follow this link you can see what tool you need.
once the pistons have been wound fully in do not touch the handbrake until you have assembled everything pump the pistons out to meet the pads it will be worth taking the vehicle for a couple of miles for the pads to bed in then you can use the handbrake this ensures that the handbrake is adjusted correctly.