Becoming a vampire
Transformation from human to vampire is described as being "the sharpest memory they have of their human life."
A human is bitten, and venom from glands inside the vampire's mouth is injected into the bloodstream. Depending upon "how much venom is in the bloodstream, and how close the venom is until it enters the heart", the transformation could last anywhere from 3–5 days, but it is interesting to note that if directly injected into the heart this could go down to 2 days, with Bella Cullen as an example.
During this time, the human will endure indescribable pain.
"The warmth inside my heart got more and more real, warmer and warmer. Hotter. The heat was so real it was hard to believe I was imagining it. Hotter. Uncomfortable now. Too hot. Much, much too hot." as described by Bella Cullen as the venom slowly circulated through the blood and eventually reached the heart.
Once the venom is injected, it is described as being "the beginning of the greatest pain you will ever understand". The venom then has to make its way all through the body. Next through the heart, and it will pump again and again in the heart until it starts meeting itself in the veins.
Then it will burn all the veins until the heart stops beating. 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 saturate every cell in the body before the process can be completed.