I often bake skinless chicken breasts in a 350° oven for about 35-40 minutes to 165°. Then, while still warm, I shred them with forks. One 10 to 12 oz. breast will make about 1 1/2 to 2 cups of shredded chicken. Chicken thighs will also work well. You don't have to do this ahead. You can mix in the freshly shredded chicken halfway through cooking in the crock pot.