Step 2: Cut the Cardboard to Size
You need six rectangles of cardboard to form the base of the folding board. The dimensions of the cardboard pieces will determine the dimensions of the folded shirt. 9 inches (23 cm) by 12 inches (30.5 cm) is a dimension that is commonly used in department stores for large shirts. Alternatively, you can just fold a shirt to whatever size that you prefer and use that as a template.
Measure and mark the outline of six rectangles of your chosen size. Then carefully cut them out with your knife. Try to keep the sides as even as possible.