Attach the ramp however you please. I ended up using a leftover piece of wood cut at 45° to keep everything stable, and another piece as a rest for the ramp.
If your plywood is fairly smooth, you may have to attach some sort of grip. I used corrugated cardboard with one side ripped off and glued to the ramp. Works wonderfully. Some leftover carpet scraps or ribbed plastic would also work well.