btw this is my first Instructable, I'll do my best to explain everything, but if there's something you don't understand or I can do better please leave a comment =)
Step 1: What You Need
sand paper/nail file
paint brush/sponge applicator
Step 2: Draw and Cut
if the bottom(base) is 3' by 3' you would cut 2 of the sides 3' by 2'(height of your choice) and the other 2 side you would cut 2' and 7/8'(3'-1/8') by 2'
of course different cardboards have different thickness, so i suggest that you draw and cut the base first and glue them together(in the next step) and then measure the thickness, then come back to this step and finish cutting the rest of the sides. and don't forget to double the thickness of the cardboard.
I hope that make sense.
okay after you got that down you need to draw and cut 3 of each piece so total there would be total of 15 piece of cardboard.
(if the cardboard is already pretty sturdy and thick, you could just do 2 of each piece, so total of 10 pieces)