I've given that advice to another user already:
For kicks, do kicks. First have a GOOD warm up to prevent injury. Kick at a heavy bag or for this occasion maybe even better a BOB. Kick relaxed without force and start at a level at which you are really comfortable with (might even be at a low kick level). Let your body accustom to the motion. Do like 25 kicks on each side, do another 25 on each side slow (meaning SLOW), then go 2" higher, same procedure. Go as high as you feel comfortable, you shouldn't feel pain. Don't force yourself to go higher, as there is risk of injury which would draw you back.
After that, do PNF-type stretching for the splits. Keep your legs warm afterwards, either by movement or outer heat (bath, sauna)
Do that every day (start always comfortable low to prevent strain trauma) and you should be able to deliver head kicks after a month.