The single row gather is made using one row of stitching as follows:
Adjust your sewing machine so that the stitch length is longer than normal (on metric machines, a length of 4-5 mm works well; on imperial machines, use a length of 6-8 stitches per inch). If you have a pre-set basting stitch on your sewing machine, you can use that as well (check the length on a scrap piece of fabric first).
Pull both threads out from the machine so that they are about 2 1/2 - 3 inches long.
Place your fabric right side up under the presser foot so that the gathering seam is slightly closer to the edge than you plan to sew your final seam - for example, if you are using a 5/8 inch seam, sew your gathering stitch 1/2 inch from the edge.
Begin sewing without backstitching - this is very important.
Sew across the fabric until you reach the other end; again, do not backstitch at the end.
Pull fabric out, and clip threads, leaving a long tail as you did when you started.
To gather the fabric, separate the bobbin thread from the top thread and hold onto the end of it. It doesn't matter whether you start on the left or right side. Using your fingertips, gently slide the fabric along the thread toward the middle - I find that this works better than actually pulling on the thread.
The fabric will begin to bunch up into small folds. The more you gather the fabric, the shorter it will become. Make sure you keep track of the other end of the thread so that you don't accidentally pull the thread all the way through the fabric. A good way to do this is to gather the fabric until the first gathers are in the middle of the section and then switch to the other side. Continue gathering, alternating sides, until the fabric is the desired finished length. You can secure the thread end by tying the upper and bobbin threads together or by putting a pin at the end and wrapping the thread around it in a figure eight.
This method should only be used on shorter sections of gathering because with only one thread, there is the possibility of breakage, especially when trying to gather long pieces of fabric.
Gathers made with a single row of stitching are a little more difficult to handle when sewing and can crush into messy pleats. I will discuss the process of sewing gathered fabric a little later.