Well, door frames and window frames of our croft are anything but rectiangular, something that has provided some experience; though not saying my methods are the preferred ones or will suit you. Hopefully, others will provide better ones.

1. Find out if the treshold or top rail is horizontal (not necessary, but makes things easier), then line out what is to be removed with reference to that piece (should create a square angle).
2. Assuming the maximum waste is at the top of the stile, make cross cuts (vertical part) for 8″, down to the line
3. Chisel carefully down to ≈1/16″ shy of the line. The stop-cuts will prevent the chips from diving to deep, but there might be fasteners.

The above allows for a saw to be inserted, by which most of the remaining waste can be removed.

4. Plane, chisel, scrape or sand down to line.

Where the maximum waste is at the floor end of the stile, things become a lot rosier: make a notch as high up as possible, and saw from there.

Experience has taught me the value of stop-cuts.

I wish I was good enough with the frame saw, as that would remove the need for step one above.