It depends on what you are trying to achieve in the way of versatility...
If you want something EXTREMELY basic and NON-CONTROLLABLE that is EASY (as in EASY) to assemble... then
http://www.skyandtelescope.com/howto/diy/3304231.html may be just what you need.
Includes a simple explanation of Ohm's law and a worded example of what it means in a practical sense.
PLEASE read the warning at the bottom... this dew heater is DC driven not AC.
OIC!