With men it is a bit more than snippity snip for guys. They cut out the internal muscles of the penis and the head, not damaging the nerve, invert it inside them and depending how big they were, may add a graft to increase the depth of the created vagina.
Women to men is more complecated. The womb, ovaries etc are removed, the vagina is mostly left intact, drugs are given to greatly enlarge the clitoris, if not sufficient, grafts can be taken from the arm and fats used to extend and or fill it out. This is just a basic overview of the basic applied principles to the operations, but if you search the net on gender reassignment, you will find all the intricate details and proceedures.
Hope this helps you..