Staple sheets of it to round or flat blocks to increase its usefulness and service life. Coarse grades of sandpaper also work well. Check the pads for fit, thin them as needed, then round off the sides. The bottom of the pad should taper so it is quite narrow, so when it is glued in place it won't catch on your clothing during a fast exit. When the shaping is done, use a water-soluble marker to mark the desired position of these pads on your seat or thigh braces.
Most people install hip pads with the top edge just below the cockpit rim.
The flat sides of the seat make it easy to get a good fit. This provides better support, but shaping the pad to fit above the seat and around the inside of the cockpit rim is pretty tricky. Most kayak seats are comfortable enough, but occasionally you'll encounter one with an uncomfortable projection in the center or a high front lip, which cuts off circulation. Use the foam to improve the shape of the seat rather than to pad it. If the seat slants backward, you can make it flatter by padding the back half only, rather than the entire seat.