Unless you're installing a cast-iron bathtub, you are going to need a mortar bed to support your tub base. Unsupported fiberglass and acrylic tubs tend to flex under the weight of water plus bathers, which can be bad news in terms of your drain staying connected and the tub base remaining solid and free of cracks. You'll need to play mason briefly and mix up some supporting material before installing the plumbing for your new tub.
Sweep the subfloor clear of debris. Dry fit the tub into place. Draw the outline of the tub base using a marker, following any protrusions; certain designs may have two low bump-outs that separate three higher areas that need more mortar for support.
Screw two-by-twos around the drain hole to act as a dam against the mortar.
Measure, cut and install any needed one-by-fours called for in the manufacturer's instructions as supports for the tub apron. It makes sense to complete this task now, in advance of setting the tub in the mortar. These one-by-fours may go under the apron against the framing, or the apron may screw directly into the framing -- making one-by-fours unnecessary.
Mix a 60-pound bag of cement mortar or nonshrinking grout in a mortar pan or wheelbarrow. Add enough water to create the consistency of peanut butter.
Pour the mortar or grout immediately into several low piles within the marked outlines on the subfloor. Expert plumber Terry Love suggests not smoothing out the piles; he says to just lower the tub into place, and the base of the tub will move the mortar around sufficiently so it isn't too high or too low.