You mount it underneath, and you raise the cage up with spacers, wood, foam, whatever to create an airspace and so you can mount your thermostat probe under there (if you use a thermostat, you should, or at least a dimmer, which has no probe).
The best way to mount heat tape or heat pads is with Velcro dots. This way you can remove it easily if needed, or reposition easily. If not using velcro, use aluminum tape. You can find it where they sell ducts at any hardware store.
I think Ultratherm heat mats from www.beanfarm.com are way easier to use than heat tape, plus they are already sealed for you. I would just use those with the Velcro dots and you are good to go.
Do not mount heat tape inside the cage, especially for heavy, ground-dwelling, humidity-loving boids like bloods.
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Mark