What kind of witch is she?
The "experimental"-type? "Curious" type? "Biologically & ethically heretical"? "Kicked out of biology-class"?
The “fleshcrafty”-type? The type who killed a squirrel and cut it up to see what it looks like on the inside? Yes, that covers it. She likes money too.

She is a witch of [REDACTED], God of Flesh, Carver of Wounds, Prince of all Erythrozyt (Leukozyt too) and others.

When "bodily"-subject matters come up in conversation our hardwired brains tend to be drawn towards Gods & Godesses of Lust & Sex & Fertility, which is a very reductive view because the body is more than its urges. Much to the disdain of [REDACTED] who falls to the wayside.
She is a witch of the Master of the Biological as an Object, a flesh witch, an artisan using organs and muscles for fun & profit.
A not-false stereotype is that those witches tend to be the studious type, learners who know obscure lore and are unhinged enough to try out the weird rituals they've just read about in a frowned upon book. Fleshgod isn't so popular nowadays.

What's her name?
(Take a biology book and open it on a random page. Pick the first medical term, that's her.)
Example: Ciliare Ganglion

What kind of wand does she use?
A Bone would be pretty boring, she thought, and went for the small intestine. She cut it into one long piece, had it rot, had it dry, wrapped it around a wooden staff. It doesn't smell so bad now thanks to washing the staff in rosewater.
But an intestine alone is not enough, she grabbed hold of an eye and plucked it atop. It's not fully fixed, it falls off when she makes sudden movements with the stick.
She's not sure if it looks good and asks for second opinions.

What's in her cauldron?
That she cooks with blood is a given. Other than that the usual array of organs, muscles, herbs & spices. Her potions are resource-heavy. It takes an adult human's entire bloodbank to fill it up.
Really, she's more for the operating table.

What kind of familiar?
It .... used to be a dog. She didn't want to decide between a cat, a dog, an armadillo and a bird so she combined it?
Take the brain of a dog, put it into a cat, replace the cat's back with an overgrown shell of an armadillo. Now rip the cat's nose off and put a big tucan beak there.
Still has the personality of a dog though. Loyal, loving and cuddly. It's called Freddy.

Where does she live?
She made herself a nice home in the sewers of [insert your setting's city]. It takes about 20 minutes to get to the nearest hospital and 30 to get to the graveyard. Both places where she gathers her resources.

What does she look like?
Remember the girl you used to know in highschool who went into biology or chemistry (maybe that's you though?). Imagine her and roll on your favorite mutations table thrice and reroll any results that would penalize her. Only beneficial or cosmetic mutations. She made those changes herself after all, why would she want to end up worse?

Example using this big mutations table (a great resource to remember):
https://coinsandscrolls.blogspot.com/2018/01/osr-1d500-biological-mutations.html
3d500  = (403+20+293)
Hinged Head. Enormous toothy grin, ear to ear. Head opens like a box.
Signal Pods. Two tentacles grow from your shoulders. The tips glow like matches.
Vertical Eyelids. They blink sideways.
You go girl! You can be an eldritch curse if you want to be!

When you meet her (1d8):
1: Scolding her pet Freddy.
2: Looking for the eye she had at the end of her staff. It fell off and must be lying around here somewhere!
3: Carrying two cut up corspes in meatbags (bags made of human leather, looks Prada)
4: Gathering resources out of a corpse and putting it into vials.
5: Washing blood off her hands by a small river.
6: Fixing up her flesh-golem.
7: Reading a medical book and laughing vehemently.
8: Sitting at a sculpting table, forming a creature out of meat. Her flesh-golem cries.

Her wares (1d6):
1: Friend in the Making (60gp): A vial with a tiny Flesh Golem the size of a baby. Grows every three days outside the vial. Gains one additional HD then for up to 9.
It will call you its mommy regardless of your gender. Ciliare offers customizations but those cost a little extra depending.
2: A potion of "Fuck me UP" (free for every purchase or 1gp on its own): You get to roll 1d3 times on a mutation table at your choice! Your referee needs to agree to the table though.
She likes to put these drinks around town, just to see what happens.
3: Customize your Body (price depends on surgery): She will listen to your request and do her best (though she will freely customize your demands and twist your words). The referee is encouraged to read Franken Fran for reference.
4: Organs of all Shapes & Sizes! (Roll a d100. Higher price indicates she doesn't have many, vice versa for low price)
5: Potion of Live Fast, Die Young (30gp): Drinking the potion will grant a creature advantage on all combat-related rolls but they will die in 2d6 hours from Caffeine-poisoning.
She tried to sell it but it never caught on.
6: Pets (50gp per pet involved): If you bring her a few live animals, she can cobble them together and hardwire their brain to love you. HD equal to animals involved. They will live for 2d20 minus the number of creatures used.
If 0 or less you get your money back and a discount of 10% on the next surgery.
For the small fee of 25 gp you can get a kill-sound for your pet. A random, small instrument (bell, a certain chord on a guitar or flute, etc) will dissolve the tissues of your pet and it loses 1 HD per round in painful screaming and hurling.

Rumours surrounding her (1d6):
1: I used to know this girl, smart and friendly. I wonder what happened to her after she got kicked out from medical school?
2: A few patients were cut open in the night and had their ailments fixed. Surgeries perfectly done. The witness can't tell who it was but they recall a woman's voice whispering "Fucking amateurs work in this place." Their stitches resemble the Fleshgods Sigil.
3: I know this nice lady from downtown who fixed my arm when I got hurt, wanna see? (*pulls up right sleeve revealing a short, tickly tentacle)
4: A weird combination of human & animal can be seen on the middle of the street. It will run to its owner, former customers of the witch who couldn't agree on the animal they wanted.
5: Two identical persons sit right next to each other. They are not twins but a couple who decided to undergo surgery to more closely resemble each other. Their brains are linked too and one finished the sentence the other started.
6: A pair of witches talking shit about this perverse lady who calls herself a Fleshwitch.

Spells she invented (1d4):
All to Flesh:
Level 6 spell.
This spell causes an object or creature to turn into a pile of flesh with sentience on a failed save. The stats change to that of a Gray Ooze.

Turn Weird:
Level 3 spell.
The receiving end of this spell needs to save or mutate. Also works with mundane objects.

Fusion:
Level 6 spell.
Two creatures standing next to each other need to fail their save or fuse with each other.
If both creatures fail, the spell is succesfull.
If one creature fails, the succesful creature will not be hurt but the other will turn into a Gray Ooze.
If both creatures manage their save, nothing happens.

I give you Life!
Level 6 spell.
One object is granted a primitive sentience. A reaction roll may be required.
On a 1 in 4, the object hates its existence.