For dogs you need to part the hair starting between the shoulder blades and apply the medicine directly to the skin. For large dogs you need to part the hair in several locations down the spine to make sure the protection covers the whole dog.
For cats you need to apply topical flea medicine at the base of the skull. If you apply it too low the cat could actually turn their heads around and try to groom the medicine off. You’d be amazed how flexible our cats really are!