It is very simple. Find a large rock that is flat on one side. Then take one large stick, which will keep the stone up, one small trigger stick, and one round rock (in the picture I am using an acorn).
Place the large stick vertically, lift it up a bit, and under it place the small stick, and under that, the rock. Now balance them so when the trigger stick is touched, the whole assembly will be destabilized and fall. The bait should of course be placed on the trigger stick before assembling the trap.
If you can not find a nice round rock for the trigger, take whatever rock you can find, and this time place the large support stick at an angle, constricting the trigger assembly the same way as above. The stick being placed diagonally, makes the trap less stable, and compensates for the rock.
Please keep in mind that these traps, just like all deadfalls are very dangerous. They are as likely to fall on your hand while you are setting them up, as they are to fall on top of some animal. Great care should be taken so a survival situation does not become even worse.