Place your flower pot onto the saucer. Flower pots have a hole in the bottom and you don't want your sand to pour out through that hole so from this point onward the pot and saucer have to be kept together. Fortunately the saucer will remain cool no matter how hot the pot gets so you can handle the pot by the saucer, but please, be very careful.
The pot needs to be nearly half full of sand. The sand insulated the bottom of the pot from the heat so you can set your flower pot grill on surfaces that won't take the heat. Of course you need to keep flammable materials far away from your flower pot grills.