Ooooh---once again you have pushed my favorite button.
I learned the slider from a Yankee pitcher who threw a very good one, and here's what he told me: "Throw it like a curve, but roll your wrist, don't snap it." He showed me the grip he used---very much offcenter, index and middle fingers very close together with the middle finger just touching one seam, fourth and fifth fingers curled up on one side of the ball, thumb underneath for support. When he demonstrated the wrist action---first slow-motion and then at normal speed---it was rather like a chef flipping a pancake or a crepe, much easier than for a curveball; then he handed me the ball and said softly, "Go ahead---try it." I got the hang of the pitch in about ten minutes, but I was well aware that one doesn't master that pitch overnight, and so I worked at it and refined it for some eight months, and the first time I had occasion to use it in a game(in a relief appearance) I got a bunch of strikeouts with it. When I added the crossfire, the pitch was just about unhittable! It became my strikeout pitch in short order, and it served me very well for the 22+ years I pitched.
You can change up on it if you wish---I preferred to use the hard slider, with or without the crossfire, because I had much better control. And sidearm---which was my own natural delivery---that was how I threw it. (Thank you, Ed Lopat.)