You decide to start off pretty simple. You set it so that you'll turn into a girl version of yourself, with all the feelings, memories and knowledge you would have if you'd lived as her your whole life. After setting it all up, you strip off your clothes and stand naked in front of your mirror, then go to the computer and click Apply.
Once you click the button, things go hazy for a moment and you see something in your mind's eye. An ovum surrounded by sperm aiming to fertilise it. You feel a kind of connection to both one of them and the ovum, and realise that they are what your body formed from. But the sperm that you feel connected to is surpassed by another that breaks through to the ovum, and as the ovum is fertilised by it and forms into a xygote, a weird sensation spreads through your whole body.
You move over to the mirror, feeling as if you're tripping, and rest your hands against it as you stare at your body- and before your eyes it shifts and alters. You feel pliable, like invisible forces are reshaping you, it's surreal but also nice. Your face sculpts into a different configuration, your jawline softer, neck narrower and lacking a bulging larynx, your lips more puffy. Your muscles and figure contort and turn from firm to lithe, and from thick and straight to more slim and curvy. Your skin turns smoother and almost all your hair either shrinks or vanishes. Even your hands and feet seem to have shrunken to fit your proportions turning more small.
Now your most obvious masculine traits start to turn into feminine ones, as the sex chromosome of the sperm that fertilized you takes full effect. Arousal clutches at your body as a warmth wells up in your chest and crotch, and your nipples not only turn sensitive and erect, but also much larger and puffier. Your sexual desire is centering on them more, as you imagine how good it would feel to have them rubbed and pinched and licked and sucked. Soft mounds of flesh start to grow from them too, and you cup them eagerly, soft pants and whimpers escaping your lips as you pleasure your emerging breasts. Before long they've swelled to full C-cups.
Just one part of you is stopping you from appearing and feeling totally female- your erection practically burns with hardness between your legs. Now this too transforms. Your penis's hardness does not abate, but its size does, and it seems to grow more and more sensitive as it recedes into you to form your clitoris. You feel an intense aching emptiness in your crotch, and your testicles pull up inside you one after the other. A split second later, you feel yourself be castrated as what had been your balls reshape into your ovaries and slide into place inside you, while your scrotum shrinks up to be almost flush with your crotch and a vaginal opening emerges at its centre as it splits to form labial lips.
As all of this has been occurring, you gradually adapt mentally to your new self. You adapt to your new gender- you think of yourself as she rather than he, female rather than male, a woman and not a man. Not only that, but you feel a strange sort of acceptance of relinquishing your masculinity, as your body adopts femininity quite comfortably. You even remember your girl self's name- Phoebe- and a whole bunch of knowledge you don't have as your old self, from people you know who you didn't as Phil to advice you were told to stay safe as a teenage girl.
Your eyes slowly focus and you look at yourself in fascination, realising you really have turned into a girl version of yourself. You wonder what to do with your new self...