AKA bench bleed.

Mount the m/c in the car, but don't hook up the lines.

Fill the reservoir.

I have long arms, so I generally open the front door and reach around the hood and plug the m/c holes with my fingers and pump the brake until the air is out...fluid squirts a little (helps to have a rag to catch it and wash the area quickly afterwards), then hook up the lines, tighten, and bleed the entire system.

Otherwise you'll have to have someone else press the brake pedal while you hold your fingers over the holes.

BTW, if you have ABS, do it without the system turned on...I dunno how much pressure the pump kicks, but it's generally a BAD IDEA to mess around hydraulic pumps...I've seen too many digits lost through hydraulic injection of various fluids, generally gangrene and/or amputation. After the system is put together you can then bleed it.