Thank you for asking about why and how. As a breed fancier it's my responsibility to say the right every time someone asks. You are starting out right, I hope that you will fall in love with the breed and help other people as other people helped each of us. It's not always fun to tell people things that they don't want to hear. But you wouldn't be asking if there was a good chance that he was breeding material.
The simple reason why is it's not worth it to you. It changes behavior in your pet w/ what benefit? The pros do not outweigh the cons.
A bitch should have an experienced finished stud. These are not expensive. There are many of them available. A bitch does not have to be finished to breed. A stud should be finished. If you want to breed him, you should finish him in confirmation. Even though some hobby breeders have a couple of bitches of their own that they breed and their own stud dog. It still makes sense to use an outside stud. It's going to give you better results. You'll find that the stud dog that they have wears a belly band all of the time. He's humping things. All he thinks about is having sex.
What if someones is willing to pay you a stud fee or a puppy? Is it worth a couple hundred dollars to have a dog that is always hiking on things or humping things? How about if they offer you a puppy? What if the puppy isn't anything like what you like in a puppy? I don't know what your yorkie looks like. But what if it's huge? What if the puppies have luxating patellas or any numerous breed issues? What if they offer you a puppy and the bitch only has one? What if your dog gets an STD or gives one? Do you know the health history of his pedigree? When you breed, you need to know all about the genetic line. Even just having dam & sire doesn't tell you nearly what you will learn from a couple of generations.
What if the bitch attacks him while breeding? Do you have time to take care of the bitch while she conceives? What if something happens to her while in your care? It's just not worth it.
When dogs are not neutered their urine smells terrible. Why do you want to breed him? Let talk about this. Whatever the reason, it's a normal one that is understandable.
Do you have other yorkies? Tell us about him. If you want to get some info to learn more about breeding just say so. There are several people that will help get you started. I'm not trying to tell you to breed. But if you want to breed, I like that you want to be responsible about it.