Ascertain the type of buildup on the pan and choose an appropriate cleaner. For example, dark marks are likely just dirt buildup and will require an all-purpose cleaner. Yellowish buildup is caused by hardened minerals and must be treated with an acid-based cleaner. Vinegar may be sufficient, or you may need a stronger, commercial preparation. Deposits with a green or bluish cast require the use of a bleach-based cleaner.
Rinse the tub. Spray the cleaner on the deposits, or apply it with a sponge, and allow it to penetrate for fifteen minutes.
Scrub the pan, if necessary, using the rough side of a scrubber sponge. Rinse the cleaned sections as you go, to prevent the dirt from drying and readhering to the pan.
Rinse the entire floor.