I am quite new at breeding but if any information changes or a correction I will correct it.
Once your decide you want to better the breed then how would you go about starting? First take a few months weeks or years if you have to, to memorize or learn the shih-tzu standard. Yes some breeders do not know the standard and just breed. This isn't the way at all. You breed to make the breed better and more closely to the standard. Once you have memorized or learn part of it keep learning it. Read as many books as you possibly can about shih-tzus or what ever breed you are interested in.
As a breeder its best to focus on one breed. Breeding 3 or 4 different breeds is a No No. You will have to focus your attention on them and their welfare. Breeds like the shih-tzu take great care. Brushing, combing, grooming etc. If you decide to breed more than one then good luck. Breeding more than one is challenging. I wouldn't if you breed 2 different breeds but breeding more than that is way too much. Gives a signal to puppy buyers who accually do research that you may be a puppy mill, very greedy about money, or a animal hoarder. I may self stay far away from breeders who breed 3 plus different type of dogs. 9 Times out of 10 The puppies they sell aren't in very good condition. In rare cases you will find breeders who actually do breed more than one and take care of all of their pups.
Once your decide on a breed now its time for your to find your male and female. If you find a male make sure that male has champions in his bloodline. Registration also matters too. No one wants a dog who isn't registered. Their are so many different types of registration. Most common here are AKC and CKC. AKC is the main one you will need to focus on more. They allow you to enter your dogs in competition. They also have a strict standard on every dog that is registered with them. CKC is very different. They will allow you to register any dog as long as you have proof that dog is what it is. I have even seen half breeds been registard with ckc. CkC also gives you alot of extras such as a ID for your pet, registering your pups isn't costly. Downside is no dog shows. Well their are dog shows but only in limited states and places. Most Dog Clubs will not accept a CKC dog. Which is a huge downside. I chose to do both AKC and CKC. Gives a huge variety.
Once you have gotten your dogs. Quickly go buy DNA tests before you even think of breeding. DNA is to make sure your pup is what it is. Some breeders can be very good lier. Sale your a pup that is mixed or has the wrong father. To make sure you as a owner of the dogs do not get in to any issues. Simply have all your males and females DNA tested.
When buying breeding stock for the first time. Its best to buy them as puppies. This why potty training will be easier along with other issues. You can also choose to buy them at older ages. This is also great since by then you can tell what the pup will look like at its adult age. With puppies you cannot tell what they will look like or how they will develop.
Once you have done all this all that is left is to net work. use twitter and find other breeders. Let them know who you are. Ask them as many questions as you can before breeding. Once your pups are 2 years go then the breeding can begin once you get a OK from your vet