I use the metal seam guide that Shelbie talks about. I put one of my plastic rotary rulers, put it under the needle, bring the needle down so it is just touching the ruler and adjust the seam guide by putting it right up to the plastic ruler, tighten down the screw. If you don't have the seam guide, you can purchase. Try this website (they're on vacation until June 27th):
http://www.221parts.com/