The thing they don't tell you about becoming a slimegirl is that you have to pull out your own bones. One by one, like baby teeth*.
The slime HRT loosens your ligaments and tendons (which are slowly turning to slimegoo), and the bones remain. It's something chemistry related to do with how slimegirls are silicon-based and bones are calcium-based, so the bones don't slimeify like the rest of your body.
Keratin converts like most parts of your body, so your nails and hair melts into your general goo body, but bones remain, as intrusions in your increasingly undifferentiated flesh. What's worse, slime goo is the same all the way through: you can feel on the inside just as well as the outside. So you feel all your bones, constantly, like a blister in your mouth back when you were biology and not slime (some of us have been feeling our bones since we were born, which is why we're on slime HRT).
So you pull them out. You reach inside yourself and grab hold, and pull. Your goo holds it in, as it clings to the porous bone surface (and slimegirl bones only get more porous, as the goo tries and fails to break down the bone). Slimeified tendons stretch and eventually break, and finally the bone pulls free, dripping with bits of your go. The experience is somewhere between excruciating and orgasmic, in different amounts.
But you have to do it bone by bone by bone. 206 bones. It takes a while. Your body slimifies at different rates, so you have to keep pulling on different bones to see if they're loose enough to come out yet.
A mature slime girl could just filter herself through a fine mesh or screen and leave the bones behind, but by definition a slime girl with bones isn't yet mature. Being able to liquefy to that degree takes time to learn, and a key step on that journey is removing your bones. Your viscosity and density is controlled by your mind, but having your bones still in you keeps you from fully going liquid. Some slimegirls even temporarily lose all cohesion when they finally pull their final bone, as they realize they're no longer human, and no longer biology. Usually they manage to pull themselves together within a few hours, so it's recommended to let him stay that way unless they're in some immediate danger. It helps them adapt to their new form, psychologically.
BTW people sometimes ask questions about why slimegirls can't digest their own bones, when they can clearly (no pun intended) digest other bones: We've all seen a slimegirl eat a chicken whole.
First of all, this is an offensive question. Every slimegirl transitions at her own speed, don't rush them!
But the technical reason is because of the immune system. Parts of it remain active and get incorporated into your morphogenic matrix, and they recognize your own body as "you". So the crepulius filaments don't try to break them down into digestible particles, like they do with other bezoars.
Slimegirls often get help removing some of the larger bones. Femurs, for example, are very difficult to remove on your own.
The skull usually goes last. Once the slimeification process is complete, your skull is protecting nothing, just more slime. Slime girls don't have brains (no jokes please, it's juvenile), every cell contributes equally to cognition. Your skull pulls free with a loud sucking sound, and the last bits of your former body pours out the eyeholes and rejoins your new form.
Many slimegirls keep their skull around as a memento of their former self. Even more of them crush it as soon as they can. Good riddance to being a solid. You're free, in a way that someone trapped in a flesh body could never be. And you're never going back.
* unfun fact about baby teeth: they loosen and fall out because of the adult tooth behind them pushing them out. If you have no adult tooth back there, or too many adult teeth behind them, you can keep your baby tooth for a very long time. I had one of mine until I was in my 30s.
