Lace the skirts and draw them tight. There are threads and descriptions on how to do this, but it's basically using a waxed shoe lace after drilling/punching a series of holes on each side of the skirts, threading the shoelace through and tightening it. Then tie off the lace. This will cause the saddle proper to "hump" a bit (maybe), and if so, loosen the tension nut a bit.

Lacing generally cuts out any "flaring" that widens the bottom when you sit on the saddle. Otherwise, not the saddle for you.