I was contacted about doing an oval inside casing and trim for an 100 yr old house. She had a few "companies" come out but none would take the job I think due to the shape and lack of skills. I'm not claiming to have done this to a specific guideline or "politically correctness", but is built good. This may be a little lengthy but I'll try to give enough info for others to improve on my mistakes if any.
The hardest part was making a true template due to it was a brick deep back in the wall. I cut a piece of kraft paper to fit the brick hole, anchored in place and the then rubbed with thumb the existing interior jamb edge to cause a crease transfer on the kraft paper to which I retraced with a marker. NOTE: mark top and bottom/up and down, which side is in...valuable for correct fit!!
I found the easiest way to get centerlines is to fold kraft paper into itself/half lengthwise matching opposite end marks . While still folded, fold once more towards the narrow way aligning the fold. At this point you should have a 90 deg angle with a qtr of the curve showing.
I laid template on a workbench than covered with a thin plastic cover (not shown in pics). This prevents any glue from sticking to template later.
Knowing this is my most interior line I cut/mitered 4 1/2" Tall/wide x 2" thick jamb pieces around template letting each joint be on the original line (not pictured) This was quiet a bit of adjusting without a protractor for angles. Once I had all my jamb precut and placed I used my kreg jig and glued up 2 halfs . I joined the halves together with NO glue for future disassembly for sawing and sanding (several times).
I then taped pieces of transfer paper around perimeter and taped template in place. I then transfered original line AND a 1/4" inset cutting line.
I then backed one of the end screws out and slid over bandsaw and re-attatched for sawing.