Decorative stitches don't do much for the structure of your garment or item, but they do look pretty. The topstitch seam does help a bit, but only by holding the seam allowances in place. It isn't strong enough to be a seam all on its own, so you'll have to start with a plain seam. It is stronger than the hand pick seam, but it won't give much support or structure to your project.
Start with the aforementioned plain seam. Press both seam allowances to one side. Don't bother with trimming them. It's unnecessary for this decorative stitch. Make sure your seam allowances are flat and that the seam itself is well pressed. The seam will look better for it.
Begin stitching on the right side of the fabric. Stitch ¼" to ½" from the seam, making sure to catch the seam allowances while you stitch. Don't miss the seam allowances. Go slowly if you need to or even stitch a little closer to the seam if you must. The whole point is to secure the seam allowances.
This seam is quick and gives a little something extra to your projects.