Start with a plain seam. This is where most decorative seams start. Press the seam allowances all to one side, making sure they lie flat and there are no unexpected creases that might cause you to miss a stitch. After securing your thread to the wrong side of your fabric, hand stitch a series of small backstitches down the edge of the seam allowance on the right side of the fabric, making sure to catch the seam allowance in your stitches.
You can space the stitches from ¼" to ½" apart. If you want a more uniform look, pin a piece of graph paper to the right side of your fabric to create a guide for yourself. If you want a more random look, forget the graph paper and space the stitches as you like.
This stitch doesn't provide much structural support to your projects, but it's not meant to. Decorative stitches are just what they sound like. They're just for looks. A fun way to put your stamp on your projects. You can make the stitches large or small, spaced evenly or not. It's entirely up to you.