you can put the cement board against the wall studs, you could put
it over the old drywall , but if the drywall is bad tear it out , and
dont put up new drywall and cement board over it,its not neccesary. its
not really much more work to take out the old drywall. you only need
cement board under the tile , drywall on the parts that dont get wet.
use the special screws for the cement board.if your house is over 30
years old and hasnt been redone you probly have drywall under the tile.
a masonry bit will dril tile,its hard to get thru the glaze but then it
goes on thru, i dont use lube on a small hole.. lucas