This is the method my vet uses and the method I would call "the best, and most peaceful".
Please share experiences here though....
The vet will make sure the rat is unconscious using the isofluorane gas placed near their face using a mask or something along those lines, the vet then checks blink reflex, I always request that they are under as deeply as they would need to be for surgery, if not, deeper.
"The injection is given in the lower right side of the abdomen, with the vet being careful not to get it into any organs (as that would cause pain)."
This doesn't take long.