You use your thumb and place it right before the cloaca and apply pressure, then you place your other thumb and index far enough on the tail and start rolling in towards the cloaca. (this require proper pressure also and placing your finger far enough on the tail, if not you will not be able to pop efficiently)
My best advice is always to find someone local (fellow snake keeper) or at a show willing to show you once someone does you will get a feel for it and the only thing you will need is practise.
Popping is the only thing I do, I have a set of probe and I can count on one hand how many time I have use them (the animals I probed were not mine and were VERY tensed adults)
Yes Adult gain more muscle control making it harder to pop them however it's not impossible just a little more difficult especially if you do not practise enough.It does seem though that some BP's learn how to hold it in when they're older, and those seem difficult to pop. Just IMHO.