If you know how to remove the keys, that would be helpful to do a more professional job. If not, contact cement will work, but you need to be real careful not to drip any glue down the sides of the key, or else it may stick to the key next to it. If you can put weights on the neighboring white keys, that may help to drop them down, so you can see alignment better. Make sure the front of the black plastic is flush with the front of the wooden keystick. On pianos with wooden sharps, titebond is better. It is better if you can sand the too of the wooden key, but may not be possible without removing. That previous thread where I mentioned contact cement was more about replacing the white key tops, but since your sharps are plastic, that is ok here too. Do not use CA glue for key tops, and never use water based glue on real ivory.
