Measure the distance on the door from the bottom edge of the door to the lower edge of the bottom hinge.
Add 1/2 inch to the measurement you took in Step 1. Mark this distance above the floor on the hinge side of the closet door frame.
Place two 1/2-inch thick shims on the floor, positioned so you can set the door on top of them and have the door in its open position with its hinges in position on the door frame. Check the height against the mark you made.
Screw the hinges to the door frame.
Test the closet door by closing it. It should close and open without jamming or rattling.