The challenge: make a healthy human female look like this:

No problem.
My first concern was securely attaching a pair of prosthetic arms to her back. Unless they had a model that would let me drive screws into her shoulder blades, there was no way I could glue them on that would be guaranteed to hold up the severed head and the cup she's supposed to be carrying. At least not if she's planning on walking around all day on the show floor at a convention.
After some careful consideration, I decided that the best option was to make the chest prosthetic as well. That way I could make the breasts, collar, and extra arms into a harness that could be strapped on with no risk of having any slippage that might result in unanticipated nudity on the show floor. As an added benefit, the prosthetic breasts would counterweight the extra arms.
With most character builds, I focus a lot of time and energy on the parts that will get the most scrutiny. This is usually the head or some particularly iconic prop. In this case, it seemed pretty obvious where most of the attention would be drawn to. It wasn't going to be her eyes.
The rest of this article is safe for work and family-friendly, but be forewarned: there are pictures of an attractive, young, topless woman being coated in purple goo. If you're okay with that and you'd like to see the step-by-step process involved in making a girl with a 32B bra size into something more like a 32F, read on...