You use a box or a can, cut the top 1/3 off and put a light sensitive photo paper inside. So the paper is taped to the inner side of the can, whilst you make a whole onto the opposite side of the can, hopefully ending up with the pinhole camera.
After the paper carefully placed inside, taped at the top to hold it at a fix position, we close the can and tape it around to seal it from water, humidity etc. It is that simple.
But I have never made one myself yet, but definitely will in the future because solargraph is a lot of fun!!