How should I secure the solid insulation to my floor? 

Ensure the area is clean and free of any debris.

The boards are lightweight and easy to cut. Secure the insulation to the whole floor area using a flexible tile adhesive (the same adhesive used to lay ceramic tiles).

Spread the adhesive on the floor with a castellated (notched) trowel, lay the insulation boards directly on top and press down firmly to ensure good adhesion.

There should be no gaps between the boards (any gaps can be filled with the adhesive). Allow the adhesive to “cure” before fitting the mats to the boards.