Refill the automatic transmission fluid on most vehicles by pulling the dipstick, inserting a long stem funnel in the dipstick tube, and adding the fluid. Only use the transmission fluid the manufacturer recommends, and avoid overfilling the reservoir. A pint of fluid takes the level from the add mark to the full mark on the dipstick.
Check the fluid level by parking the vehicle on a level surface and letting the engine idle. When checking the fluid level, check its color and odor. It should be bright red and have a petroleum smell. If it is dark or has a burned odor, it is time for draining the fluid, changing the filter and adding new fluid.
A low fluid level usually indicates a leak in the system. One of the more common leaks is in the line that connects the transmission to the radiator. Once you locate the leak, fix it.
Changing the transmission fluid is a messy job that many owners choose to delegate. However, it is important they find out what the process involves. Many facilities attach a machine to the transmission cooler line to dump the fluid and then refill the unit with new fluid without a filter change. Protect the transmission by asking for a filter change, and repeat the process every 30,000 miles.