On floorboards is fine. Personally I go for a wet mix of mortar (4 or 5 parts sand to one cement) and aim for as close to full coverage as possible (put mortar down and create ridges such as you see bricklayers do to allow the mortar to spread into the troughs as necessary). Make sure you have a spirit level and get the tray rim level - use small wedges to hold up the exposed corners overnight while the mortar sets. Don;t be too frightened to tap down with the palms of your hands - unless you're some kind of bodybuilding freak you'll be unlikely to break the tray! Ideally not much more than a 10mm gap around the perimeter works well as it allows space for floor covering to just go into.