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).
Run power cable through side of box.
All ready to go, Mr Bond..