Remove the sole plate. Saw through the 2x4 sole plate at the inside edges of the jack studs. Remove the cut portion of the plate.
Community Q&A
1. Can I do it without using jack studs?
Never, the jack stud is what the header rests on and is meant to carry the weight.
2. How do I fix an out of square door frame?
To fix an out of square door frame, it would be easier to adjust the door at the hinges if you can.
3. How do you trim a door frame if the opening is all the way to the ceiling?
I would just install a 1x4 right to the ceiling.
4. Wouldn't it be easier to fit a small sole plate to each side of the frame, thus doing away with the need to cut off the center part of the plate later?
Using a single piece of wood ensures the bottom is square and you will get a better seal.