How do humans transform into monsters?

With the current level of technology, this is impossible. If you count people with some kinds of genetic disorders as "monsters" then a change in the genetic code is all that is needed. As for an already-living human turning into something like a dragon, this continues to be pure fantasy The closest that someone can get would probably involve a bionic exoskeleton (something likely to be developed considering the rate of technological advancement) covered with a flesh-like material and colored to look real.