In most cases your new slavegirl will be an adult woman, readily and wilfully presenting herself for enslavement as deliberate fullfilment of self-determined identity.
The defloration of children into womanhood is not a topic I claim any pre-eminence in ; nevertheless, find below a modest suggestion of presentation.
The only needful item of scenery is a generously ample full length mirror, such as she can comfortably observe her transformation reflected back at her as it occurs. Handcuff her wrists behind her back (though a leather wrist-to-elbow cuff works much better, if available). Put a sturdy doggy leash on her to hold firmly on to (if you're not the more primal sort that prefers holding firmly on to a fistful of the animal's occipital hair) and drive the point home -- or rather, carve the point a new home out of her fleshy, visceral moist.
Once that's surfeited to your satisfaction (but in any case before she actually starts enjoying the ministrations -- there'll be time enough for that later) anoint her puckered butthole with a dollop of vaseline and force the blood of her erstwhile maidenhood into her rectum. I don't say do it suddenly, nor should you, but on the contrary -- do it slowly, steadily and taxingly, such as it may last her the remainder of her life.
As resistence melts and the sphyncter turns to milking your rod from where it was vainly attempting to deny it entrance, bring her to her knees, and bring her blood and hard earned guts into the back of her throat, so she can thoroughly taste herself, and develop an appreciation for just what she's made of and just how far it can take her.
Aftercare should consist of cuddling, while she whisperedly recounts for your benevolent amusement the subjective highlights of her recently expired childhood ; optionally elder slavegirls extracting uncounted, palpitatingly exhausting orgasms out of her worn and bloody apparatus all the while.
Or at the very least -- that's how we do it.