There are little gadgets that you can add to your sewing machine that allow you to sew in a straight line and/or at a specific distance from the edge of the fabric. These gadgets are called seam guides.
Many of our sewing machines come with them, but they can be a real pain to use. It's time consuming to reposition them and take them off when they are in the way. You can buy after-market guides, but they can be very expensive.
I create my own seam guide inexpensively using a roll of old masking tape. The video below will show you how it's done.
If you want to learn more about seams and how to construct them, visit the Sewing Seams page.