Pretty easy, assuming your frame has the cuts to accept it. You'll need the lever and the detent/spring. I wasn't able to find a video on installing a thumb safety on a gun that didn't have one but I was able to figure it out easy enough from videos about removing the safety. I've done it twice now and I've got two more used M&P's on the way that I plan to do it to. I sold the M&P40 I had that didn't have a notched frame and was able to win an M&P40 2.0 for $50 less than I sold the other gun for.