Conventionally, back stitching is the very first and last thing you do when stitching a seam. Backstitching is the act of reversing stitching to lock the stitch and thus prevent the finished seam from becoming loose or unraveling. Every sewing machine has a backstitching lever or button located on its right front side. When this lever is pressed, the sewing machine stitches in reverse.
To backstitch a seam by machine: Insert the needle at about 1/2" down form the top edge. Insert the needle and drop down the presser foot.
Holding the reverse button down, press on your foot pedal to stitch backwards until you arrive at the seam top edge. It may take a few tries to comfortably reverse stitch in a perfectly straight line. In this case, practice makes perfect!
Next, release the reverse button and stitch forward, following the seam's designated seam allowance guide as you would regularly.
When arriving at the bottom of the seam, stitch until the very bottom, at which point hold the reverse button down again and stitch backwards on top of the existing stitch for a distance of about 1/2". For added strength, and a more convenient/fast way to cut the loose threads, stitch forward again until you arrive at the very bottom edge of the seam.
When it comes to backstitching, it is important to keep the reverse stitching overlaying perfectly with the existing seam's stitching. This will ensure that the seam allowance lays flat and the backstitch doe snot cause any tension or distortions at each end of the seam. As you practice more, you'll be able to achieve even backstitching by force of habit alone.
Note: some fabrics cannot be backstitches on both ends of the seam as they retain too much tension. These fabrics are often more lightweight featuring sensitive fibers (such as silk). In this instance, you should only backstitch either at the beginning or end of the seam, leaving one of the end un-backstitched. You can then iron out any of the tension created in the machine stitching process, through the end that is not locked with backstitching. Once ironed, you can seal the open stitch end with a hand-applied backstitch or by tying the loose threads together.