Most "Pantherophis" probe deep as males. The deepest female we've probed (not GP but with corns) has gone in 4 scales. One may even have been 5, but my memory's not clear on that one. But males are usually a good 7 onwards. Always make sure to probe both sides to be sure.
As for which way is the easiest, follow Mark's advice.

lol Or best thing to do is have someone experienced show you. But then again, I never had anyone show me, so I will tell you what I go by. First off, always have some lube.

lol! Dun wanna irritate the inner linings. As for what size probe. I usually go with something smaller than 1/4 of the length of the anal scale/s. That is, I line the probe up against the length of the snake's tail, and check the thickness of the tail with the thickness of the probe. My opinion is, it's better to be smaller. Some may disagree, but if you're gentle enough ( which you should be!) you're not gonna pierce anything. Maybe someone else may have another way of doing it, but always remember the #1 rule, never use force! I've never had a need to really push. You should be gentle enough that you can use, say, the tip of your fingernail to push it in. Best to always have a second person hold the snake while you probe.