Running stitch can be used to hem the edges of a project, attach two pieces of fabric together, baste waste canvas to a project or ornament a garment. If you are using it to hem the edges of a tapestry or sampler, please review the mitered corners tutorial linked here.

Cut a 12 to 14-inch length of embroidery or sewing thread, and thread it through a sharp needle. Knot the other end. If you are using regular sewing thread, you may want to double up the thread.
Decide how long to make your stitches and the space between them.
To begin, bring the needle up through the back of the fabric to the front at the right end of the line to be worked (point 1).
Take the needle down from front to back a stitch length from the first point to complete a single stitch (point 2).
Bring the needle up again from back to front a space length from the end of the previous stitch.
Continue stitching in the same manner, spacing the stitches at regular intervals and keeping the stitches uniform in size, until you reach the end of your line.