Warning: there are things that can go wrong when you jump start a car that can be hazardous or even deadly. Do not attempt to jump start a car unless you are aware of these precautions.
That said, in most cases, it's pretty easy. From About.com's auto repair site:
The first thing to be right about is the order to connect the cables:
Attach one end of one cable to the dead battery's positive terminal.
Attach the other end of the same cable to the positive terminal of the battery in the starting vehicle.
Attach one end of the other cable to the negative terminal of the battery in the starting vehicle.
Attach the other end of that cable to the engine block of the car with the dead battery. Look for unpainted metal surfaces and be sure it will clear anything moving when the car starts. Do not attach the negative cable to the dead battery itself.
Got that? Dead + / Live + / Live - / Dead engine block. Some jumper cables will have those numbered.
Edmunds.com has a jump-start page with instructions and video
Step Five
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.
Prevent Blindness America has a page of precautions when jump-starting a battery:
1. Check your battery for cracks, corrosion, and loose wires every time you change your oil.
2. Always wear a pair of splash-proof polycarbonate goggles when jump-starting your battery. Your goggles should have a Z-87 label on the frame. This means your goggles are certified use during auto repairs.
3. Never smoke or use anything that may spark when working on your battery. Car batteries contain hydrogen and oxygen, two gasses that could ignite and explode.
Once the car has started run it at fast idle for a few minutes. Assuming that its charging system is up to snuff, this will charge the battery sufficiently to restart the car after the next step. To make sure that you aren't injured I recommend that you now turn off both engines so that you can safely remove the cables without getting tangled in the moving parts like fan blades and belts. Remove the cables in the reverse order that you used to hook them up, taking the ground cable clamp from the engine metal first. After removing the cables immediately re-start the car. If you feel comfortable removing the cables with the engine running have a care about loose clothing and fingers and keep the cables away from moving parts.
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