How to tell apart my male and female guppy fry?

Several websites tell me that you can tell their gender at 1 wk old and that they will breed their "litter mates" between 3 and 6 wks old which is why females must be separated when you can tell them apart at 1 week. Well no one tells you HOW to tell them apart!! Anyone got pictures or can tell me or anything?? I want to separate the females so I will have virgin females for my Turquiose male.