Step Two: Make a pair of marks
Next, I make a pair of marks from the top edge of the MDF on the inside casing trim guide lines. Depending on the opening width anything from 6 to 12 inches will work.
Step Three: Place curved trim onto layout
I place the curved trim molding on my layout with the inside edge hitting both marks that were just made. Then I draw a line where the inside and outside edges of the trim molding intersect the straight lines that represent the side casing.