If you don't already have one, you can buy one here
Remove self-adhesive backing and stick heat mat to underside of lid.
The heat mat warms gently from above using infra-red. Placing the heat mat in the base is not recommended as it drys the kits out too much.
Place thermostat anywhere in box, set at correct temperature for the species of choice by turning dial (28 C is good for tropical species).
Plug in heat mat. The probe can be placed anywhere but ideally in the centre of the box (on top of kits).
To add power cable groove (this will have been done for you on later models).
Place a square of tinfoil over the edge of box (this keeps your screwdriver clean). The preferred place is over the existing small square hole.
Carefully! heat near the end of a medium/large screwdriver shaft (20 - 30 seconds over a gas stove works fine).The screwdriver should have an insulated handle.
Once the screwdriver shaft is hot, push with medium pressure directly downwards over the tinfoil to make a groove in the plastic.
Repeat 3-4 times until the groove is below the lip of the lid.
Leave the tinfoil in place and allow everthing to cool. Remove the tinfoil. Any excess plastic can be easily removed with pliers.
Run power cable through side of box.
All ready to go, Mr Bond..