Easiest way would be to use flexible pad heaters, wire-tied around the bottom of the trans.
- The cons to this though are the fact that most trans cases are ribbed for strength and therefore would prevent optimal surface contact of the pad heater.
- The easy part (relatively speaking) would be the installation of the pads and wrap the wire around to secure it/them.
Best way would be to have a circulating style heater and physically heat/circulate the trans fluid.
- Great way to heat up an auto trans considering most of them have at least 5 quarts (some have 14 or more quarts) of fluid in them and already have lines running to a cooler in the radiator that you can tap into for the heater.
- Not so great for a manual trans though (I know that Honda transaxles only house 2.25 quarts of fluid).
So an experiment would have to be done in order to determine if heating your particular vehicle's transmission would be viable.