Technique #1: Sherman fold
Use roundnose pliers to grasp the edge of an annealed copper panel and twist. Repeat to grasp and twist another section of the metal adjacent to the first twist. Continue to work your way around the perimeter of the metal, and continue twisting sections [PHOTO 1].
Place the panel on a bench block and use a rubber mallet to flatten it and compress the folds [PHOTO 2]. Anneal, quench, and dry the metal.
Repeat the process, twisting the metal from different angles. The more you repeat the process, the more dramatic the wrinkled effect will be.
Keep in mind that the more wrinkles you make in the panel, the shorter it becomes. I repeated the process five times; the length of my panel reduced from 8 in. (20.3 cm) to 6 in. (15.2 cm). The width stayed approximately the same, but some curvature occurred.