All you do is line up the edge of your foot with the numbered edge of the ruler and if you have an adjustable needle, just move it over until it is over the hole for your preferred seam allowance and then slowly lower the needle (without thread) with your hand wheel. If your machine does not have an adjustable needle, just slowly lower the unthreaded needle until it is in the hole of your preferred seam allowance and check for a landmark on your foot. If you don't have a "landmark" use a fine tipped sharpie to mark it.
The Sewing Machine Seam Guide is about 3.25 inches long by 7/8 inches wide. It has holes for 1/4", 3/8", 1/2", 5/8", and 3/4". And you can use the 45 degree angled end when you are piecing your binding together.