How to fix a door jamb
Remove the door strip and hardware off the door jamb
Remove the architrave that is covering the door jamb
Remove the door jamb
Prepare for the new door jamb
Prepare the packers for levelling out the door jamb
Nail the top and bottom packers for the door jamb into place
A quick way to work out how many packers to use is to pack out from the wall to the notch in the top plate. Then nail them in place where you've just marked. Now place your spirit level between the top and bottom positions, and pack out the bottom until it's level. Once you have enough packers in place, nail them into position.
Install the new door jamb
Nail the middle packers for the door jamb into place
Check the jamb is sitting correctly before reinstalling the door strip and architrave