Clean and Prepare the Tub
No matter how well you apply the new enamel, it won't adhere properly if there's caulk, dirt or grime under it. Use a razor knife to cut out any old grout around the border of the tub. Scrub the whole tub thoroughly with an abrasive cleanser. Remove or tape off the faucet, drain rim and any other fixtures. Tape off the walls and floor around the tub. If any of the cast-iron understructure of the tub is exposed by design (i.e., a claw-foot tub in which only the well is coated in porcelain, with the outside presented in cast iron) tape off and cover all the non-porcelain areas.
Acid-Etch the Surface
Even if the enamel surface of your tub is already worn and dull, it all needs to be de-glazed before you can proceed, or the new coat of paint won't stick. Use the acidic etching agent provided in your kit, or from your home-improvement store, to eat away the gloss of the porcelain. The etching agent goes on like paint, and you leave it on for about 15 minutes, then thoroughly rinse. Wear thick rubber gloves and goggles when applying the etching agent.
Prime and Paint
Put on a respirator. Use your paint sprayer to apply the primer, which provides a bonded surface for the new enamel paint to adhere to. Spray it very lightly, starting from the top and working your way down into the tub, making sure not to let any drips form. (The primer is darkly colored so that you can see when you've got full coverage against the white porcelain.) Apply two coats, waiting 30 minutes between them.
Spray your topcoat of white paint-polyurethane mix in the same manner--very lightly, just covering the surface, then waiting 30 minutes and coating it again. Spray on at least three coats of topcoat, and as many as five, to get full, smooth coverage.