There should have been instructions with the tub. If not, then go to the manufacturer's website and check it out. Usually, it is attached at the edges to the studs and just sits there. It is much better to set the fiberglass tub in some wet sand mix mortar to fill in the nooks and crannies underneath it. This keeps the bottom from flexing. Don't walk around In the thing until the mortar sets - make sure that it sets level in left-right and in-out, or it won't drain properly. My unprofessional experience...