Q&A for How to Make Cardboard

If you use many pieces of paper, yes. The more pieces there are, the thicker it gets, resulting in harder paper.

Always start with a large piece of cardboard. In the middle, you'll have the layout of one square in the middle and four squares on each side of your first square (forming a plus sign). Choose one of the sides and then add an additional square, which will serve as the lid. In the end your box should look like a cross. (Make the measurements exact so the box comes together properly.) Then, I would recommend scoring each line before folding the box.

It depends upon what you want to put on the shelf, and the thickness of you cardboard. You can't put any liquids that might spill on it either, that would ruin the cardboard.

This is the fastest and most logical way. The only other way to do this is commercially, which is by using a machine and you'd be unlikely to fit such a big machine into your house!