Extend a tape measure along the length of a floor joist.
Make a pencil mark 3 inches in from one end of the joist. Place one additional mark every 12 inches until you reach the other end of the joist. Ensure the last pencil mark sits within 6 inches of the joist’s other end.
Set a length of fiberglass batt insulation between the marked floor joist and the joist next to it. Push the end of the insulation batt tight to the front of the joist cavity. Hold the insulation in place with your non-dominant hand.
Align one end of an insulation hanger rod with the pencil mark. Hold the rod across the insulation batt. Push the other end of the rod against the second floor joist of the cavity. Keep both ends of the rod roughly 1/2-inch from the bottom of the joists.
Twist the insulation hanger rod until the middle peak sits against the face of the insulation.
Work along the insulation, by holding it in place and installing an insulation hanger at each pencil mark. Place one hanger on either side of a connection by two pieces of insulation.