How can I make a chair pad for my piano bench out if old pillows

I have done this in the past using a worn, flattened pillow that wouldn't fluff anymore. I would secure them in an old pillowcase and my kids used them as floor pillows to lounge around on. I've also used a chair cushion (the kind where the back is connected to the seat) intended for outdoor chairs for bench seats. Measure your surface, find the pillow you want to use. You don't want it too puffy or it will be hard to sit on. Using fabric that will wrap around your pillow with an inch or two to spare on each edge (I like sheets and pillow cases, but, you could use upholstery fabric). Sew the fabric right sides together (or use fabric glue if you don't sew), leaving one end open to insert the pillow. Turn right side out, smooth the pillow into place, fold in the ends that aren't closed and either sew or glue together. If you want to be able to wash it without the pillow, insert a zipper or velcro so that it is easily removed. Hope this helps. By the way, if the pillow is too fluffy for what you want to have, you could remove some of the stuffing.

To answer your last question, yes. Either elastic, velcro or strings you can tie that will all be hidden from view when the piano bench is closed will be the only way you will keep the cushion from sliding off constantly. If you didn't want to do that, you would need to remove the top of the seat from the bench and attach the fabric you use for the seat to the bottom side of the bench with staples...more permanent and not easily removed if the fabric needs cleaning.

To prevent it sliding off a rubber/plastic shelf liner from the dollar store would work too. Put it beneath the pillow and on top of the wood.