Jumper Cables are a "must have" item for every car.
Knowing how to use them, then would also be necessary.
Here are a few things to keep in mind:
Never allow the two cars to touch.
Never allow the jumper cable clamps to touch while any clamps are attached to the battery.
Wear eye protection if you can.
ALWAYS connect POSITIVE to POSITIVE and NEGATIVE to NEGATIVE.
Keep the engine in the good car running.
Ok, Here is the sequence:
Attach the POSITIVE cable (the red one, or the one with the red stripe) to the POSITIVE post on the battery of the running car.
Attach the other end of the POSITIVE cable to the POSITIVE post of the dead battery.
Attach the NEGATIVE (the black one) cable to the NEGATIVE post on the battery of the running car (the good battery).
Attach the other end of the NEGATIVE cable to a clean unpainted spot on the engine of the car with the dead battery.
Attempt to start the car with the dead battery. You can rev the engine slightly on the running car while the other car is attempting to start.
Keep both cars running and turn on the head lights of both cars.
Disconnect the cables in the reverse order of how they went on: NEGATIVE cable from engine, NEGATIVE cable from good battery, POSITIVE cable from dead battery, POSITIVE Cable from good battery. (Don't let the cable clamps touch till all are disconnected)
Turn off the headlights
DON'T stop the car with the dead battery. Let it run for a while, preferably at highway speeds to charge the dead battery.
Attach the red clamp to the Positive terminal of the dead battery (step 2)
Attach the red clamp to the positive and black clamp to the negative terminals on the good battery (step 1 and 3).
Finally attach the black clamp to the engine of the car with the dead battery. (step 4)
Some explanations for the reasons on some steps:
Cars use the metal frame and engine itself to complete the electric circuit back to the battery. The negative terminal is wired directly to the engine. That is why you can connect the negative terminal to the engine. That bolt where the battery connects is a good place, usually to attach the negative cable. This is why you don't want the cars to touch, you don't want to allow the current to pass from one car to the other that way.. you want it through the cable.
When batteries charge, especially when they are charging quickly, they split some of the water in the battery into Hydrogen and Oxygen. The Hydrogen is explosive, and a stray spark from you connecting or disconnecting a cable can set it off. This is not a very likely hazard, but it is one you don't need to mess with. That is why the last cable is attached to the engine a little ways from the battery, where there is not likely to be any explosive concentration of Hydrogen.
The solution in the battery is Sulfuric Acid. That is why you should wear eye protection if you have it. Today's sealed batteries are not likely to leak, explode or spew it out, but why chance it.
Turning on the lights before disconnecting the cables allows any voltage spikes to have a circuit to expend themselves in, lessening them and helping to protect other electronic components in the car.
John Wlodarczyk of Ottowa Canada adds:
A set of cables that have sturdy, strong clamps, a minimum of 6 gauge wires, and are long enough to reach from the front of a car to the front of a car PARKED BEHIND it, about 12 -18 ft., should be the MINIMUM considerations when purchasing cables.