How do you teach parrots to poo only in their cage?

Teaching a parrot to poo only in its cage (instead of on you and your furntiture) is possible, but it is tricky. It's not like teaching a cat to use a litter box or teaching a dog to do his business outside. Potty training cats and dogs relies on using their natural aversion to fouling their dens. Parrots don't have that kind of instinct because in their natural environment, when they poo it just falls to the ground and they don't have to worry about it.

Potty training a parrot depends on getting the parrot to realize when he needs to poo, and understanding that you only want him to poo in one specific location. It might sound like a good idea, but it has its drawbacks. If he things you only want him to poo in his cage, he might make himself very ill by trying to hold in his waste if he gets a new cage, if he's in a travel carrier, if he's at the vet, or even if you move his cage to a different location. There's an Excellent book on parrot training by Mattie Sue Athan that gives a detailed explanation of how to potty train a parrot, as well as explaining some of the potential drawbacks of potty training a parrot. The book is called "A Guide to a Well-Behaved Parrot"

You might have better luck getting your parrot to wear a bird diaper when he is outside of his cage. It takes a while for the bird to get used to it, so be patient when you introduce it. See the related link for a company that makes diapers for pet parrots.