This is a subject NEAR AND DEAR to the hearts of all responsible dog breeders. To begin, with breeding is not for everyone. Just because you have, a dog does Not mean it Should be bred and most should NOT be bred; especially with our GIANT breeds. (I breed giants)
There are many aspects of breeding dogs, the average person does not know. Learning takes years of studying to become a GREAT breeder and it takes a GREAT mentor to help facilitate this process. Knowing exactly what you are doing is not trial and error but years of research. You must research both the Sire and Dam along with the littermates on both sides many generations back.
Along with all the research on your dogs, You MUST do ALL health testing required on your dog (or dogs if you have a male/female).
Then there is the true conformation of the breed. You REALLY need to know this because many people out there have what they say is an AKC pure Bred but is not. Just because the dog looks good does not mean your dog should be bred to it, just because it has a good personality does not mean your dog should be bred to it. Many dogs are be registered AKC but THEY SHOULD NEVER be bred. Just because they have papers does not mean they are BREED QUALITY!
You ALWAYS want to breed for the BETTERMENT of your breed AND THAT IS THAT!
Personality, temperament, and health are KEY. Passing down a terrible temperament to a bunch of puppies' guarantees, they will end up in a shelter or on in a ditch somewhere.