The hole for poop is fairly obvious. The hole where a girl pees from is too small for your penis to fit into. The vagina is between the two.
It's about an inch above where the poop hole is. If you're having trouble finding it, use your finger first. This will give you a chance to figure out where it is and what angle is best, as well as giving the girl a chance to get used to the sensation of something in her vagina.
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