Start by stitching a plain seam with wrong sides together. The ideal seam allowance here would be ⅜". This gives you a little space to work with and makes trimming the seam easier. Once your plain seam is complete, trim the seam allowance to ⅛". Press the seam open with a warm iron. Then fold the fabric right sides together, making sure to fold directly along the stitching line. Press well and precisely. This is the key to a good and well sewn French seam.
Stitch ¼" from the fold, totally enclosing the seam allowance you previously trimmed. If you've done your job correctly, you shouldn't seem any of the raw seam allowance on the outside. It should all be enclosed.
On very lightweight fabrics like Swiss batiste, you might want to make the size of the seam even smaller. If you can trim the seam allowance even more and stitch ⅛" from the folded edge if you can manage it. This way you are sure to minimize the appearance of your seams. If you can't pull this off, don't worry. The standard French seam should work well enough.