To bred a teacup dog, breeders pair the smallest dogs of the litter (runts of the litter) together. The health of these dogs is not important, only thing that matter is their size. Another way how you can bred a teacup dog is malnourish the puppy, which is againg, causing a lot of health related problems.
Teacup dogs are also too fragile for this world. They can be injured while playing with you or other dogs and they can even injure themselves by jumping down from the sofa.