A fabricated door jamb will be in three pieces that you will have to assemble before you can put it in place. Once you have the replacement jamb in one piece, put in into place in the door opening. Temporarily screw the jamb into place on the hinge side first. Try to get one with LOSP timber treatment.
This should ward off any more rot. It could also be that the door itself is out of alignment. In otherwords it appears to be closed but in fact its not putting enough pressure on the mechanism to tell. If your going to try and do this yourself then I would suggest you get a tape measure and measure the door across and lengthwise and if you need to draw it out on a piece of paper so there wont be any confusion that will be fine as well. The Home Mender, Dustin Luby, shows to to replace a door jamb after it had been kicked in!
Adjusting pre-hung door jambs to match the thickness of old walls. To replace door casing, such as brick mold: Use a utility knife to cut through the paint and caulk where the casing meets the door jamb , siding, and miter joint. Step-by-step instructions for installing a new door jamb and adding decorative trim.
Over time rainwater can damage and rot the door jamb and brick mold on an exterior door. Replacing a Slider With Hinged Doors. I purchased these door jambs to replace on that is rotte and the other one is split down the middle due to a heavy storm door , and the door being blown open by strong wind.