To successfully overwinter turtles a deep pond with a large surface area (best exchange of oxygen) is necessary. A hibernating turtle needs at least a foot of water that isn't frozen at the bottom of the pond. The pond must also have a large surface area, to facilitate sufficient oxygen levels in the water. Aquatic turtles usually settle into the sediment at the bottom of a pond to hibernate and switch to absorbing oxygen through the skin, so the levels of oxygen in the water must be kept high enough (see below).
Since hibernating aquatic turtles require high oxygen levels in the water, some method of adding supplemental oxygen is a good idea. Running a pump (to keep water moving) or air pump can increase oxygen levels in the water.
Water De-Icers and Heaters
Pond supply companies usually sell submersible heaters which can be used in the pond overwinter. De-icers will usually prevent the pond from totally freezing over, which can help with oxygenation as well as make the pond safer. A temperature (water) around about 50 F seems to be the best for hibernation (you'll want to avoid heaters that raise the temperature much above this as a cold and inactive turtle that is not at a low enough temperature to hibernate is likely to be severely stressed).
Someplace to Dig
As mentioned previously, aquatic turtles in the wild usually submerge themselves in sediment at the bottom of a body of water for hibernation. You must make sure there is something on the bottom of the pond such as a layer of leaves to give the turtle something to dig into for hibernation. Some people even put a pan of sand or sand and soil mixed at the bottom for hibernation. Too much decaying plant matter can negatively impact water quality though, so be careful about having too much.
Prepare the Turtle
Only healthy strong turtles should be hibernated. Turtles should also have been in the pond since at least mid summer to have time to adjust to the changing season to prepare for hibernation, and should be older than 6 months. As the weather cools the turtle will eat less and less (this is normal). As the turtle stops eating and the temperatures approach 50 F (10 C) stop feeding.