Flip flops are the only thing standing between you and the dirty ground. Dust, dirt and grime combine with moisture from puddles, rain or foot sweat and leave a filthy mess on your flip flops.
Flip flop stink is caused by bacteria mixing with sweat. The bacteria breeds and gives off a bad smell. You can remove the cause of the odor by killing the bacteria.
Baking soda is an excellent stain remover. Dip a wet toothbrush into some baking soda and start scrubbing. Continue the process until the entire flip flop is clean.
Putting flip flops in the dryer could melt or otherwise damage them. Air dry, preferably in direct sunlight. The sun has disinfectant properties and can help kill bacteria