are you talking about thoughs upside down plate like terrocotta pots?
if so i had problems at first but then ended up just giving the area i wanted a hole in a nice whack with a small hammer (not to hard) and then sanded the edges smoothe with sand paper and i think a bit of water (dont entirely remember)
if so i had problems at first but then ended up just giving the area i wanted a hole in a nice whack with a small hammer (not to hard) and then sanded the edges smoothe with sand paper and i think a bit of water (dont entirely remember)