This is an old popular mechanics article on building a homemade leaf spring crossbow. Man I wish popular mechanics still had articles like this. But since as I write this my Facebook is covered with warnings not to eat laundry detergent, I imagine their readership has changed.
This article is pretty straightforward and it has lots of clear pictures and good instructions.
It contains information on an all metal action bow with a walnut stock, as well a plastic actioned crowwbow with a white pine stock.Additionally, it shows how to build crossbow bolts for this device, because who makes a crossbow and doesn’t plan on shooting it?