Just something from someone with experiance and training.
YOU CAN NOT LEARN VENT SEXING FROM A BOOK.
Period.
I was taught by an expert and can sex almost every animal I know from birds to reptiles (Except guineas until around 6 weeks old with any accuracy). You will most likely kill or hurt the chicks and only have a 90% chance AT BEST of getting it right even with training and experiance. Why do you think that hatcheries only guarantee 90% accuracy? There's a reason.
I've vent sexed every kind of bird from bantams to ostrich and although I raise 300 - 500 chickens a year, I don't even vent sex my own. It's just not worth the risk.
As someone else pointed out, if you really must know the sex of your birds at a day old, buy sex-links.
As far as all the magnetic sexers, pins on strings, picking them up by their heads, lines and circles, etc... I've found that eenie meenie minee moe works just as well and is twice as fun.
Just sayin'
Take it or leave it.