Your naughty girl or guy has one place they do not want to be spanked - right on their bottom hole. It isn't wise to spank too much there, maybe 50 or 60 whacks at the most, but you can make it a fun game (for you).
Some good implements are rubber spatulas, a smooth, round hairbrush or maybe the last 12" of a belt.
Put your naughty girl on the bed, bent over a stack of pillows, or maybe over the back of a couch. Tell her to reach back and spread her cheeks wide and to hold them open no matter what. Ler her know you are going to give 10 or 15 spanks right on her hole.
If she lets go of her cheeks or fails to make her hole available for spanking, start the count over. It is very hard to take 10 or 15 in a row, so you might reach the 50-60 spank sanity limit. If that happens, just give her a long spanking on the rest of her bottom and tell her you'll try the bottom hole spanking again in a day or two and will continue to repeat it until she can hold her cheeks for the whole spanking.
