Step 1: strip off the Ultracote from the razorback area. Feel the sides to determine where the convex fuselage either flattens or turns concave, and mark those locations with a Sharpie. Use a straightedge to connect those marks, then mark another line about 1/2 inch outboard / downward, then cut the Ultracote at that line.

Step 2: cut through the sheeting at the first line you made in Step 1, and peel away the sheeting. Try not to completely destroy the sheeting, as it will come in handy in a few steps. Notice the spine that's built from T-shaped plywood and a balsa cap.

Step 3: remove the spine. Do not destroy the spine in the process, as you will want to use it to make a new spine later on.