I don't think vent sexing is easy at all, that is just my opinion. I'm sure you can find a website that will show you how to do it, but A) It can hurt the ducklings if you don't know what you are doing. B) I, personally, think it is difficult.
I think a person just has to have a natural talent for it. I have raised ducks for many years and can't very accurately sex ducklings. If it were easy, more than a couple hatcheries would offer sexed ducklings.