Dirty dishes and glasses must be placed in the kitchen sink before they can be washed and returned to stock.
Wash dishes yourself
Place dirty tableware in the sink. Aim at the sink, hold the interaction key (left-click) shown on screen, and select Wash. Clean items are returned to your available stock.
Hire a dishwasher
Unlock the Dishwasher at Tier 1 in the Expansion app, then recruit one from the Employees app. When their task is enabled, they wash tableware that reaches the sink automatically.
Tip
A dishwasher only handles the washing step. Waiters or players still need to clear tables and bring the dirty items to the sink.