Shimming a Door Jamb
From the back of the door, shim it. Put the shim just underneath the top hinge and check the margins. Shim and check the level on the top. Then check for level along the side of door that opens. Now on the lock-side of door, pry the jamb open a little with prybar and put in a shim so the gap is equal down the entire length of the door that has the knob opening. Last, shim the gap so it's even. Now the new or replacement door is level and opens smoothly. You're ready for nailing.