Easiest way is to use compressed air. What you do is place a piece of wood between the pistons and the outer portion of the caliper where the pad goes. Your doing this to catch the pistons from completely flying out of their bores when you stick the tip of the compressed air gun into the bleed hole to force them out. DON'T use to much pressure. Start off slow. It doesn't take much.
When you place the calipers back on the car the bleeder valve most always be on top, not at the bottom of the caliper. This is to allow air to be bleed out when you bleed the system. If the bleeder is on the bottom of caliper then only fluid will come out and air will stay at the top of the caliper when it comes time to bleed them.