The Slim Jim is a thin strip of metal with a notch cut into one side at the bottom. The notch is intended to hook the rod which attaches the door handle to the door lock mechanism, and pull it into an open position, thereby leaving the lock itself undamaged. This is a popular tool amongst locksmiths themselves and inexpensive for the consumer to purchase from any local hardware store.
Locate the rubber seal between your door and door window that prevents debris and rain from getting inside. Take your wedge and place it in between the rubber seal and the window, creating a gap which is large enough to get the slim jim through. You can use anything from a stick to a matchbook. Leave your wedge in there, roughly in the middle of the door, and grab your slim jim.
Slide your slim jim into the door gap you just opened. In between the door handle and the door lock, there is a thin rod connecting them which you want to hook with the notched end of the slim jim. The rod is closer to the locking mechanism at the edge of the door. In older cars, their isn't much else in the door. If you have a hard time hooking the rod at first, try twisting your wrist a little going down and up, feeling it out.
Once you've hooked the rod, pull up or push down on the rod to rotate the mechanism into an open position.
