Push the needle up through where the blue circle is
And then go back down through to the left of the blue circle where the yellow circle is
When you come back up through make sure you are a small distance away from the first stitch so that there is enough room to go back again.
Put the needle back through to the left of the blue circle again. Repeat these steps to make lines of stitching. If you prefer you can make the lines much tighter by leaving less of a gap between each stitch.