I am afraid I haven't had much luck with foils and cabochons.
I tried gluing the foil directly onto the cabochon, but it wrinkled and looked a mess. I know others had success with this but I didn't :sad:
I then made a decal (slightly smaller than the cabochon surface) onto a nail form. CND Shelac, base coat, cure, colour, cure, foil glue applied foil. Then onto the cabochon I applied a coat of Mother of Pearl, cured, then positioned the decal onto the tacky MOP. Then to the back of the decal I applied top coat x2. Ensuring that I coved the area larger than the decal to help seal it. This method worked (see pic), but it was a lot of faffing about. There may be another method I am not aware of.