Running stitch is a simple linear stitch (meaning it's worked in a straight or curved line).
Fasten your thread on and bring your needle up from the reverse of the fabric where you want to start stitching. Take your needle back down to create your first stitch. Bring your needle up again, keeping the distance between each stitch the same as the stitch length. Keep working this along your design. You can vary the stitch lengths to get different effects.