The first thing you want to remember when jump-starting a car is that a slight possibility of explosion does exist. This is because hydrogen gas--which forms as a battery discharges and loses its fluid--is flammable, and a spark from the battery cables could, theoretically, set it off. We emphasize slight because the gas would have to be fairly dense around the battery for this to happen. This is unlikely unless the battery has been sitting for a long time and little or no air circulation has occurred in the area, but it is possible.
That said, I should add that in the last 15 years I have performed at least 50 "jumps" on various automobiles (you see, I used to own a lot of Chrysler muscle cars) and have never had a problem. One of the keys to avoiding the big bang is to hook up your jumper cables in the proper order. Follow along and we'll show you how.
Park the booster vehicle close to the one that needs to be jumped, but not so close that the vehicles are touching in any way. You'll want to use a good set of jumper cables with thick wire and clean clamps. As you are hooking up the jumper cables, make sure they don't dangle into either engine compartment where they could get caught on moving parts (belts, fan, etc.). Turn off the ignition of both vehicles, set the parking brakes, and make sure that they are in either "Park" or "Neutral" depending on whether the vehicles have an automatic or manual transmission. Also, turn off all accessories like lights, radio and, if the vehicles are in a safe area, the hazard flashers.
Begin the process by clamping one of the positive jumper cable ends (red) to the positive battery terminal (labeled with a "+" on the battery) of the dead vehicle. Be sure the connection is strong with the clamp securely "biting" onto the battery terminal.
Connect the other end of the positive cable (red) to the positive battery terminal on the booster vehicle (again, confirm that a "+" is next to the battery terminal). If the terminals are corroded on either vehicle, you may have to scrape them with an abrasive such as steel wool to achieve a solid connection.
Connect the negative cable end (black) to the negative battery terminal on the booster car (marked with a "-"). Finally, attach the other end of the negative cable to an unpainted metal surface on the engine of the dead car. Find an unpainted bolt or bracket that is as far from the dead battery as possible. This will provide a solid ground while further reducing the possibility of igniting any hydrogen gas.
Make a final check to confirm that the jumper cables are not near any moving engine parts, and start the booster car. Let it idle for several minutes, depending on the state of the dead battery. If the dead battery is new and was drained by the lights being left on an extended period of time, it will probably start immediately. If it is an old battery or it has sat for a long time (more than a month) it will probably take awhile to charge it sufficently.
Start the dead vehicle and let the two vehicles idle for a few minutes. If the dead vehicle refuses to start, don't keep trying or you might damage the starter. If there is the possibility of additional problems, like a lack of fuel, don't continue trying to start the dead vehicle until the other problem(s) are solved.
Once the dead vehicle is started and running smoothly, disconnect the jumper cables in the reverse order that they were connected. As you disconnect them, be careful not to let the dangling cables fall into the engine compartments or touch each other.
Drive the revived car to somewhere safe and secure before shutting off the engine. Depending on the battery's condition, it might need to be jumped the next time it is started. To properly charge the battery, attach it to a certified battery charger and leave it connected for at least 12 hours. You can also take it to an automotive repair shop for complete charging. Driving the car for an extended period can also charge the battery, but this should be done only if the other two options aren't available. A vehicle's alternator is primarily designed to maintain a battery, not charge it from a complete drain.