Be sure both batteries are of the same voltage.
Be sure both batteries are of the same polarity (have the same grounded terminal).
Be sure the vehicles are not touching.
Be sure the vent cap holes are not obstructed.
Do not smoke or allow sparks around the battery.
In cold weather, check for frozen electrolyte in the battery. Do not jump-start a frozen battery.
Do not allow electrolyte on your skin or clothing.
Be sure the electrolyte is not frozen.
Make certain that the ignition key, in the vehicle with the dead battery, is in the OFF position. Connecting cables to vehicles with on-board computers will result in computer destruction if the key is not in the OFF position. Turn the heater blower motor on the high speed setting.