The one thing
She brought in a warmer and put the mice inside it. Twenty minutes later, when they were thawed enough to survive the ordeal of receiving life, she brought them out again. Any longer, and its body would start rotting.
Carefully, she placed them on their bellies, and stared at them for a few seconds. She nodded to herself and got to work.
Under the effects of her Handling skills, the mice's fur split, then their skull, and finally their brain. A small hole had appeared, and she located the mice's equivalent of the lateral ventricle.
Staring at the human version, she used both Organ Handling on it. Slowly, it turned into the exact same shape as the mice's. The spots where it would connect with the rest of the mice's brain, she transformed it to become as similar to the mice's using Organ Alteration.
All that was left was to perform the operation.