The first step is to design your pattern. The bat is made much stronger by the addition of the vertical strips that are going to make up its thickness. As long as some strips go from the tip of the handle right into the body of the bat playing area, you can design them to be as decorative as you like. Copy this design by eye, or make one of your own up, or go to thedadcando
project where you'll find a free download of the template pattern.
Of course you could design yours on a computer too using any simple drawing package. When you have a design that you are happy with, mark it out on a piece of corrugate cardboard and carefully cut it out. I agonized about the direction of the flutes (corrugated card), for the best performance. A bat will not really have much lateral force applied to it, but given that you putting longitudinal strengthening, it makes sense to have the flutes running across the face of the bat. That way you'll be making the strongest composite possible (from the given materials).