I enlarged the pattern on the copy machine (first enlarging 200%, then 190%) until the scale at the bottom was 6 inches.
The waistline will be the smallest measurement across the pattern. I drew several lines straight across where I thought the waist might fall, and measured. The waist on this pattern measures 19 1/2 inches (9 3/4 per side).
To compare, the waistline on the last pattern I enlarged measured 27 1/2 inches, so this was a small corset. To expand the pattern I slashed down the center of each piece and spread them apart until I hit the measurement I needed.
I measured 4 inches below the waist, roughly the top of the hip bone, and 3 1/2 inches above the waist, the underbust, measured across, then figured out how much I'd have to reduce or enlarge those areas. I drew the new seam lines in blending the new points into the old using a hip curve.
I added an inch to the top edge of the pattern so I'll have plenty of fabric to work with when fitting the bust.
One last adjustment. I made a corset for myself and drafted the underbust line and bust apex at my real life measurements. The finished corset looked matronly, the bust looked heavy and there was overflow at the top sides. Not pretty. So I decided to slash through the pattern between the waist and the underbust and lengthen everything 1/2 inch. Hopefully this helps ward off a potential repeat of that unattractive situation.
After I true the seam lines I can cut this out, trace it off on fabric, add seam allowance, and sew the mock-up.