You ask a very open ended question. but I can get this question started. Skin necrosis occurs when there's not enough blood flow (or enough meaningful blood flow ie: no oxygen) to the skin. Aside from cutting the blood flow to the skin in question (which can be a technical error), it can be from heart failure (no pump), it can be from poor arterial supply(no pipes) ar no venous outflow(congestion). It can also occur from infection or immune rejection. So many ways for skin to die! The best advice for any patient is to make sure that you are in good health and most important: don't smoke.
If you are not in good health, do something about it. If you use any nicotine containg product, stop. Every patient carries their physical baggage with them. Not everyone is a candidate for everything. Prepare for surgery as you would for a marathon.