Click the left mouse button sand pours from your cursor and collects at the bottom of the screen. You use the C key to pick colors and the left and right arrows to cycle between colors you have used. What you create is up to you. I have drawn mostly colorful mountain ranges. I never claimed to be artistically creative, but I'm having fun anyway.
If you decide you don't like your creation you can use the E key to erase it and start over. When you are satisfied with your sand painting masterpiece you can add it to the gallery.
You can waste hours just viewing the over 500 pages of gallery entries with over 50 thumbnails per page. The art ranges from my kind of simple piles of sand to beautiful patterns, to detailed portraits and paintings, to love messages. The gallery is not censored so you may encounter a few offensive images.