They were required skills at the wilderness program I attended. Tom Browns book has a great guide to them as well (fig 4 and deadfalls). Basically you need 2 sticks, one about 3/4 length of the other.
Cut a notch in the shorter one, about 1/5 of the way from the side that you will have holding the rock up (the distance isnt a big deal, it is all in how you want them to stand) and cut the longer stick so it is pointed at one end (so it fits in the notch in the shorter one).
You will need some sort of cordage, sinew works very well, you want something strong, certainly if it is cordage get it wet while you twist it. I don't recall specifically the length needed but you can fiddle with it. You need to cut another notch around the shorter stick, this just needs to be a shallow ring around it for the cordage to fit in. Then set the 2 sticks together (with the bigger one in the notch) and see how much cordage you need in order to have it tied securely around the shorter stick and to be able to hold a very very small stick taughtly around the base stick. That is the trap, then you need a rock or whatever you will use to kill the prey and prop it up with a very thin almost miniscule stick. The trigger stick is the tricky part because it needs to prop the rock up, but also fall when an animal licks it or even sniffs close to is.