Source: tori29 on Twitter
A guy finds a free trial version of a body modification app for his phone and starts to play with it. He immediately uses the gender bender toggle, changing from male to female, intending to turn himself into a tall, slender, and handsome tomboy with short hair and a prince-like aura, intending to pick up women for lesbian sex with herself as the top.
Unfortunately for this new girl, she's not used to her more feminine hands and is a bit clumsy with her touchscreen. This means that she overshoots her intended slider target for her breast size, giving herself F-cup which are bigger than she'd like. She tries to make her bust smaller, but the adjustment sliders seem to only go one way, and that's towards enhancing feminine features. After taking off her T-shirt which no longer fits her sizable bust, she tests this, trying to change her height from an average 5'6" to a tall 5'10" to offset her larger chest, but the slider won't move in that direction; furthermore, her clumsy touch makes her shrink herself down to a short 5'0". Annoyed at her clumsiness, she tries to cancel her changes, but realizes that she can't undo anything until she finishes customizing all the options for her body, and she can't skip any of them. Sighing, she goes down the list of options and watches the results of her actions on her body in the mirror. She's only intending to make minor changes so she can get this over with as fast as possible, but she keeps accidentally increasing her femininity as she goes over every option, making her hair unmanageable in length, obliterating her muscle definition and making her frame petite and delicate, developing an extremely thin (as in, borderline impossible) waist, massively expanding her hips and giving herself a thigh gap, making her butt huge, raising the pitch of her voice to a very girlish soprano, and making her facial features far more on the "cute" side than the "sexy" side.
She's relieved that she's nearly done, but with the final trembling swipes of her dainty pedicured fingertips, she accidentally increases her new body's sensitivity, increases her submissiveness, increases her libido, and makes herself fully heterosexual (as in, no longer attracted to women and fully attracted to men). Horrified at the severity of her femininity, she tries to go through the body modification process again, only to learn that free users can only adjust their body once a day. Paid users can get past this, but she balks at astronomical upgrade price of $3,000, which she has no way to afford.
This means that she's left stuck in the new body for 24 hours as the cooldown refreshes, but as she takes a selfie of her new ultrafeminine form for posterity, she really takes a look at her new body. It's not what she wanted, but... wow, she's really cute, with a blend of a young girl's childlike innocence and a voluptuous woman's sex appeal. As she stares at herself, lewd thoughts run through her feminized mind, and she wonders what else she can do with those new hands of hers, and whether or not they'll be clumsy with what she has in mind...
Please use Samuel as the guy name and Samantha as the girl one (if it comes up), and besides the main focus on the sensually vivid details of her experiencing this transformation, give some focus to her feelings, the feelings of anger at herself for being clumsier, annoyance and being upset that she's not getting the body she wanted, horror and shock at how much she's changed when she's in the final stages, and then finally realizing that being extremely girlish and cute has its own allure, with erotic curiosity getting the better of her judgement.