Step 1: Tools and Stuff, You Will Need...
Foil (from your sandwichs perhaps)
Device you want to turn on.
Step 2: Constructing the Switch.
Fold it in half
Take the foil (I folded it in half for strength but a strip of tape would work better) and attach to one half of the card with blutac or double sided tape. Try to place it so that it receives the mostr pressure from the bulldog clip. the metal needs to be exposed. leave enough hanging out the side to attach the croc clips to. Repeat on the other side, making sure that when folded, the touch and that the overhang is on the other side.
Step 3: Making the Anchor Line
Make a loop around your card and insert that into the clip. That should hold it all together a lot better.
Step 4: The Trip Mech
Tie string through hole (the longer trip wire string)
Make sure card is wide enough to prevent any shorts within the trip switch.
Step 5: And Finally...
Croc clips attach to the foil pads and from there, to anything you want. I don't recomed the Quick and dirty Squib for inside use but the choice is yours.