Here’s how I install a pre-hung interior door. First, I measure the height of all three hinges. Then I mark their locations on the hinge side of the rough opening. I’m using 2 ½” long 15-gauge trim nails. I put two shims at each mark. These shims are wedged shaped, so as I slide them closer together they get thicker and further apart they get thinner. With a 6-foot level I make sure the shims are perfectly in line with one another working from the bottom to the top, this gives me a plumb Swipe or use ↑ ↓ to keep watching.