Always connect positive first.
If you do this, when you connect the negative cable you will not get a spark. If you hook up the negative cable first, you will get arcing when you connect the positive cable.
If you don't believe me, do some research. Better safe than sorry.
Here is the full proceedure:
When jumpstarting a car, consider safety first. If at all possible, wear safety glasses. Also, never smoke or use matches while jumpstarting a car or handling a battery. Batteries can release explosive gases.
First, make sure that the cable connectors to the dead battery are not damaged, broken or have frayed wires. This could also be a reason that the car is getting no power, since a charge cannot freely pass through damaged cable ends. If this is not the problem, follow these ten easy steps to jump start a car properly and safely:
(1) Move car with the "live" battery as close as possible to the car to be jumpstarted without the cars touching.
(2) Switch off power to both cars and make sure that they are in park. Set the brakes.
(3) If any corrosion is present, scrape this off with a small knife or other object. Do not let corrosion come in contact with bare hands. If possible, wear safety gloves.
(4) Connect the metal clamps of the red cable to the positive posts of each battery. The positive posts are marked "+" or "POS". The negative posts are marked "-" or "NEG". Make sure you do not allow the metal clamps of the red and black cables to touch each other.
(5) Connect the clamp from the black cable to the negative post on the car with the live battery. Connect the other end of the black cable to bare metal on the frame of the car with the dead battery, the grounding connection. This is safer than connecting the black cable to the negative post of the dead battery.
(6) Keep the cable clear of the fan and fan belt at all times.
(7) Be sure that the ignition is off in the car with the dead battery and start the car with the live battery, revving the engine lightly to charge. Attempt to start the car.
When both engines are running, remove the cables by first taking off the black grounding connection on the body of the car and then the other end of the black cable. Remove the two red cable ends last. Make sure that no objects are let under the hood of either car before closing the hood.
(9) Do not turn off the engine to the car with the dead battery until you get to where you were going. If possible, let the car run for half an hour to make sure the battery has a full charge.
(10) An incorrect battery connection could cause an explosion so make sure to connect red to red and black to black.
If these instructions are followed and the battery will not charge or loses its charge, take car to a mechanic. The problem might be elsewhere in the ignition system.
Here is another writeup from the Battery Council of America:
- Connect positive (+) booster cable to positive (+) terminal of discharged battery.
- Connect other end of positive (+) cable to positive (+) terminal of assisting battery.
- Connect negative (-) cable to negative (-) terminal of assisting battery.
- MAKE FINAL CONNECTION OF NEGATIVE (-) CABLE TO ENGINE BLOCK OF STALLED VEHICLE, AWAY FROM BATTERY.
- Start vehicle and remove cables in REVERSE order of connections.