Is there already sand in there? If so gently push it over making approx. six sq. in. bare spots and add the sand like in this video. Putting Sand in an existing tank. - YouTube
Turn off circulation pumps, add sand and let it settle for 15-30 minutes, turn pumps back on.
If you have existing sand, I've read to stir the top layer a bit with the new sand so that you don't suffocate the existing beneficial bacteria that's been growing on the upper layer of your substrate.