How do you bathe a cockatiel?

You don't in the conventional sense - you provide a shallow (half an inch to an inch) bowl of 10 or more inches across, of water, and let the bird bath, or not. If the bird is un-used to having a bath provided, it can take ages (days and weeks) for them to work-out what it is there for. You can also VERY LIGHTLY, mist them from a hand-held mister/spray as used to mist house-plants - this also encourages them to preen.