In order to repair a rotted or partially rotted door frame, you must remove and then replace the portion where rot has occurred. The process is not too hard, though it is essential that you remove all of the rotting wood prior to adding new wood. If all of the rotted wood is not removed, you will not be able to attach the new. Before you begin, make sure you have all of your materials handy.
Mark an inch or two above and below the portion of the door where rotting occurs. Check carefully to be certain that you have not overlooked any rotting portions of the wood.
Cut out the rotted portion at the marks you made on the existing frame. Use the saw to cut through the frame. You will be able to pull the pieces of wood out of the frame once you have sawn through on both sides. Make sure you get every piece of the rotted wood out or you will not be able to replace these portions of the frame and your frame will be susceptible to further rotting.
Measure the total length of section removed with the tape measure. This needs to be a very precise measurement so the wood will fit in so be careful.
Cut the new section of wood from the matching frame piece. You should mark on the wood using your measurements, then cut in a straight, even line.
Seal the ends. Prime the ends of the new wood with Kilz or other primer to form a seal. Paint the old wood ends with Kilz to seal also.
Install the new section of wood. Apply wood glue to the back of the new wood and on the ends. Align the edges of the new piece with the existing frame. Clean off any wood glue that seeps out along edges or seams using a rag.
Apply wood putty to the seams if there are any gaps. Wipe off any excess so that the seams are invisible.
Paint the exterior door frame including the new section. This will help the frame look entirely new and prevent the new section from standing out.
You can also do this process for multiple pieces of a frame if you need to. Just be sure to measure carefully so that your frame looks whole once you are done.
Be very careful to keep fingers clear of the saw while you are working on cutting the door frame.