Use a siphon hose or pump to remove most of the water from the hot tub. Scoop out the rest of the water with your plastic cup. Soak up the last of the water with the old towels.
Spray the sides of the hot tub with a non-sudsing cleaner made for pools and spas. These cleaners lift off grease and film, but don't leave behind a residue that can mess up the chemical balance in your hot tub.
Wipe the sides with a soft cloth to remover both the cleaner and the grime it lifts off the hot tub.
Run your hand over the surface of the tub and feel for any rough or dirty spots. Apply more cleaner to these areas and let it sit for a minute. Rub the spot with a non-abrasive, plastic scrub pad. Wipe with a soft cloth.
Remove the filter from its housing and rinse it, or replace it with a new filter if your filters are disposable.
Fill your hot tub with clean water and adjust the level of the chemicals.