Lift the door off the ground. You will need to lift it high enough that each hinge half is at the same height as its corresponding half.
Place the loops of the hinge on the door half into the openings of the hinge on the other half. The loops should alternate.
Slide the hinge pin into the hole at the top of the hinge. If the hinges are lined up correctly the pin should slide in easily. If the pin meets resistance, double check that all the hinge holes are lined up.