Building a door buck (FHB #8, Door Butt Gauge) is a good idea if you find yourself dealing with a house full of door hanging from scratch, but for a quick setup, the answer is right in your toolbox. I use two bar clamps, one on each side of the door. Small blocks between the clamp and the door face will prevent marring.
—Paul A. Jones, Pacifica, CA
Edited and illustrated by Charles Miller
From Fine Homebuilding #9