I haven't used the diffusers, but used the spray itself. The vet told me to spray it in an area where my cat spent a lot of time (not to spray on the cat directly), such as a sleeping area.

That didn't work well, so I got a very stretchy cat collar and would spray a tiny bit into the collar twice a day. You need to depress the spray very slowly and carefully to do that, but it's easy enough.