Long hair cats need to be washed at least once every month.
Short hair cats, actually never need to be washed at all. Wash
only when they are dirty. Or if they have white furs, and they are
looking dirty.
The most important thing is to calm them down, with plenty of
coaxing. Prepare your towels, hair dryer, and shampoo before
hand. Use warm water, cold water is shocking to them. Their body is
small and they lose body heat fast and will start shivering. They
could easily get sick.
Wet, shampoo, rinse, quickly, and then towel dry. Then blow dry
with warm air. All them time you must, be calming the cat
down with your voice.