it's very easy, but it does take practice to get them to pop quickly.
pick up the snake support the body on your forearm or have someone hold it.
place your thumb in a way that the tip of your thumb should be right at the snakes vent pointing towards the tail.
then with the other hand place your thumb a little way past the mid point on the tail towards the tip.
with the thumb on the vent gently pull back putting light pressure, so that you open the vent slightly
at the same time with the thumb on the tail roll your thumb at the same time putting pressure towards the vent
when you do this once your thumb on the tail gets to the vent everything will pop out. if it doesn't then re-roll again.
if you have a male and know for sure that it's a male then you need to move your thumb on the tail more towards the tip of the tail. and re-roll.
many people that don't know how to pop think that you just put presure on the tail while openning the vent and they will pop. when you just put pressure on the tail you are also putting pressure on the hemi's and that's why they can't pop out.
the key is to roll from mid point or slightly past mid point towards the tail, then you put the pressure and then you roll towards the vent.
it will pop out for sure. practice a few times on one snake and then give that snake a break and move to another to practice.
probing is a lot faster and a more secure way i think of sexing.
good luck
Pitoon