This started when I wanted a cork or bulletin board to keep the random notes and lists I write from cluttering my desk. I didn't want to buy one, or look at the brown color. So I decided to make one I could choose the color of, wouldn't put holes in the wall, and out of things I already had.
I glued 4 layers of cardboard (alternating the "grain" of each layer) using a 3M spray adhesive, then used mod podge to cover it in tissue paper. I used two different colors of pink to try to make it blend in with my wall (it doesn't really, but that's ok, it's mostly covered in papers). I used frog tape (like painters tape but works better) to put it on the wall, which works great, and doesn't leave any holes.
Then I got an idea from this instructable
to put text or a picture on it to make something for my wall. Basically you just need to use a laser printer to print a MIRROR IMAGE of what you want, then cover it and the "wall art" in mod podge, stick them together and let dry a bit. Then pull the paper gently off and there will probably be paper that needs to be rubbed off. Shazam! Then I covered it in one final layer of mod podge.
The next boards I made for my mom who is a piano teacher. She wanted something that her students could put their pictures on next to the new "level" or "skill" they have learned. I used the same process of tissue paper and mod podge, toner transfer, then a final layer of mod podge.
I think 3 paneled display boards (the kind that are usually used for elementary science fairs and such) are a good source of cardboard, there are big pieces, usually pretty flat and sturdy, and don't bend as easily as some cardboard boxes.
When cutting my layers, I added an extra inch or two. Then I used an exacto knife and a metal ruler to cut it to the final size after all the pieces were glued together for a cleaner edge. Maybe a band saw would work well for this?