I don't have photos of my setup, but you can visualize. On your work surface (counter, whatever) you place your flat thing (a big tile, whatever- mine is a piece of scrap acrylic). Then, you put your drywall screen on top of that. It's like sandpaper, but more mesh-like. Then, you rub your stone on top of the screen to flatten it out. It helps to draw a series of exes or hatches with a pencil all along the surface of the stone. When you've flattened enough that the marks are faint or gone, then you're done.
You can get drywall screen at a hardware store. My local big-box store (not sure if it was Lowes or Home Depot) just had smaller pieces designed to fit into some sort of holder, so that's what I'm using, but when I use up these, I'll pick up some 8" x 10" size pieces I saw recently at a better hardware store.