Frankly it's a couple scalp prosthetics that surgically attach via lightweight medical myomers to the
existing muscle structure around the ear. Add in some hair transplant surgery and you're done; instant flickable ears that with training and fine myomer control could even express various subtle emotions. For tails, that might be a bit more functionally difficult just due to length and mass-vs-connecting-surface but again a proper segmented core anchored off the coccyx with a basic myomer controller attached to the lower back muscles should handle it. Of course, you'll have to alter your entire wardrobe for that, or just wear skirts, but hey.
Actually making the ears
work as sensory organs on a biological level is effectively impossible; you'd have to open the skull and extract part of the primary motor cortex to allow room for a proper ear canal. Not
quite such a good idea. If you don't mind going mildly cybernetic then simple level-adjusting microphones buried in the inner ear fur (which doubles as noise-cancelling material!) could run to a cochlear implant to augment your own hearing; think of it as a robust hearing aid in your new cat ears.
Science making catgirls work? I'VE BROKEN THE INTERNET!
