How much flexwatt you need depends on the size of the fridge. I would measure top to bottom inside the fridge and double that number. Run the 11" flexwatt down each side of the fridge. This will help to distribute the heat evenly. You don't want any hot spots. Be sure you know what you're doing when you cut and wire the flexwatt. There will be live electricity at the bottom that you need to insulate. You will need a heavy duty extension cord and possibly a power strip with a circuit breaker. Most importantly, you need a high quality proportional thermostat (night drop feature is not required). If your fridge is small enough, a fan may not be needed. The fan helps to evenly distribute the heat. You want the temp to be very stable. Be sure that the door seal on the fridge is good. If not, replace it. All in all, I spent about $350 setting mine up, but it is roughly the size of a full size refridgerator. The thermostat alone will cost around $100. I used a herpstat from mg reptiles.

Hope this helps some.
Steve