Be sure that both batteries are of the same voltage. Vehicles covered by this guide, and most vehicles on the road today, utilize a 12 volt charging system.
Be sure that both batteries are of the same polarity (have the same terminal, in most cases NEGATIVE grounded).
Be sure that the vehicles are not touching or a short could occur.
On serviceable batteries, be sure the vent cap holes are not obstructed.
Do not smoke or allow sparks anywhere near the batteries.
In cold weather, make sure the battery electrolyte is not frozen. This can occur more readily in a battery that has been in a state of discharge.
Do not allow electrolyte to contact your skin or clothing.