Many fiberglass and acrylic tubs require a mortar bed for support to prevent flexing and cracking. Check your tub's instructions. If required, mix a bag of mortar mix with water in a bucket to a thick, pancake-batter consistency. Trowel a 1-2 inch thick layer of the mortar onto the subfloor where the base of the tub will sit. With a helper, carefully lift the new tub, angling it to get the pre-assembled drain through the hole in the floor. Slide it into the alcove, resting the back flange on the stringer board. Gently press the tub down into the mortar bed. Use your level across the tub deck in both directions (lengthwise and widthwise) to ensure it is perfectly level. Use wood shims under the feet if necessary to achieve level before the mortar sets.