Use the eye dropper to pick a color from the color pallet. This fills the line you just made with color. Now pick another color and go through the previous steps again with the freehand tool. Keep doing this until you have all the colors you want to use. The box doesn't have to be full.
Some white space makes it look a little more interesting.
Another selection box will appear. Move this image to the lower left hand corner. Paste again placing the image in the lower right corner. Click on image and then click on resize. I make mine 1030 x 100.