The transition from human to vampire is said to be very painful, in fact it’s so bad that "The pain of transformation is the sharpest memory they have of their human life."(TW20). When James bites Bella in TW23, we witness this clearly. She compares the sensation in her hand to "fire", and words such as "screamed", "writhed" and "torture" enhance the agony that Bella endures even at the very beginning of the transformation process. As Stephenie Meyer points out, this is at the beginning of the conversion, "Painful enough, but not as bad as if she had several bites." (PC8)
Edward speaks in awe of Carlisle’s transformation, explaining, "It’s a miracle he was able to keep silent, to stay undiscovered," (TW15) due to the agonies he would have endured. Carlisle’s conversion would have been particularly painful because "his bites were not deliberate or well placed," (PC8), therefore it took a little more than three days to become a vampire. The pain is so bad that for "every minute" of the transformation, "a victim would be wishing for death." (TW20)
Alice is unusual in that she cannot remember her transformation at all. "She didn’t even seem to notice the pain, poor little creature. She’d been stuck in that black hole of a cell for so long." (TW22)
The transformation process is simple, assuming the vampire doesn’t drain the human body of blood. A human is bitten, and the venom is allowed to spread for a few days. The actual length of time a transformation takes is dependent upon "how much venom is in the bloodstream, how close the venom enters to the heart." (TW20). The venom is pumped around the body as the heart keeps beating, and various physical changes occur as the poison moves. One of the changed that happens is the body’s "healing" (TW20) from the injuries that may have occurred to it, such as Emmett’s mauling by the bear.
"The greatest pain begins when the venom is all the way through the body, through the heart, and it starts meeting itself in the veins again and then burning them dry. It moves slower than blood because it’s thicker. Each beat of the heart can only push it so far. The changing/burning process is slow. The venom has to leak through to every cell before it ends." (PC8). The final stage in conversion happens when "the heart stops"(TW20), at which point the human becomes a vampire.