Cut bacon into 1-inch pieces, and cook until crisp. Remove bacon from pan, crumble and return to the pan. Add onion, and cook until tender, about 5 minutes. Add garlic, and cook until just fragrant. Add collard greens, and fry until they start to wilt.
Pour in chicken broth, and season with salt, pepper, and red pepper flakes.
Transfer to slow cooker, set to low, cover, and simmer four or more hours, or at least until greens are tender.