Start inserting the pins. Be systematic about it, finish an entire line before you start on another; you can mess with them later if it doesn't turn out just right. Don't be afraid to deviate a little from your original pattern as you go along; sometimes when you start putting the pins in you realize that the design needs a little tweaking to make it look better. Use different sizes and colors of pins to add variety and detail. You can also get different effects and textures by changing the spacing of the pins.