To train a dog to sit in a bathtub, spend time creating a positive association with the tub by giving the dog food, treats or toys while in or near it. Pair that with a command, such as "get in," to indicate you want the dog to climb into the tub.
Bathtubs can be very scary for dogs because they are slippery. Use a non-slip mat to help give your dog more stability.
Clicker training can be useful for teaching dogs to willingly sit in bathtubs. Use the clicker to shape desired behaviors, then follow it up with a reward. Begin by clicking whenever the dog shows interest in the tub, then click when the dog touches the tub with its nose. If your dog seems reluctant, use a treat placed in the tub to lure it. Try to work on this for a few minutes every day until the dog is sitting in the tub calmly, and remember to reward even the smallest progress.
Once the dog learns to sit in a dry tub, adding water may set your training back. Some dogs find water in general frightening, while others are upset by the spray from the shower. Start with a small amount of water in the tub and reward the dog for calmness and interest. Never force the dog to tolerate bathing if possible, because it may cause the dog to become afraid.