I put the breasts in the crock pot and cook them until done, pour off any liquid and then shred with forks. You can add a variety of sauces to the shredded chicken (either homemade or bottled). I usually let the chicken cook a half hour or so after adding the sauce, so the sauce gets properly heated. If you add sauce before they cook, the sauce may burn.
I always thaw my chicken breasts (either in the microwave or for a day in the refrigerator) before adding them to the crockpot. I went through a period of time in my life where I'd just put frozen chicken breasts in the crockpot, but, honestly, they spend a LONG time at the perfect temperature for bacterial growth and toxin production to occur if you put them in the crockpot frozen. I decided I'd better stop doing it because, for me, the convenience wasn't worth the potential for food poisoning. Some of these bacteria produce toxins which are not destroyed in the cooking process, so you can get sick even if you cook very fully and kill all the bacteria because the toxins will still be there, unchanged by the cooking process. I know that a lot of people do put frozen chicken in the crockpot (and, like I said, I used to as well), but it's sort of like playing with fire. You may or may not get burned, but, if it happens, it won't be pleasant.