Use the 4th and 5th fingers on the black and white keys respectively. Try to relax and wave your hand rather than keeping the wrist stiff and rigid.
I never understood explanations that say play from the back, from the shoulders, arm, wrist or fingers. I would guess you use them all, but use them in a sense of loose rather than stiff.
Also make a video-tape of your playing and look at it and maybe compare it with other players, easily found on You Tube or other sites. A decent camera can be found at $200 and large memory sticks at $30 or so, which offer a teaching device you can easily and profitably operate.