French seams are great for finishing your edges on a variety of fabrics, but it's also especially useful for finishing sheer ones. I'm using two squares of cotton calico for this demo, but you could easily do this with a chiffon or organdy. I'll stitch with white thread so that it's a little easier to see, but you should use whatever thread best matches your project. I've sewn this using the machines at TechShop.
To begin, prewash and iron your fabric. No one likes this step, but you really shouldn't skip it if you want your project to look its best.
Step 1: WRONG Sides Together
Stitch 1/2" in from the edge. Please note, this is not going to be the finished seam allowance; we'll be taking off an additional 1/4" later, for a total of 3/4". If your pattern calls for a different seam allowance, adjust accordingly.
Step 2: Press and Trim
Step 3: Right Sides Together!
Fold your fabric back along the seam so that the right sides of both pieces are facing each other. Press. Sew 1/4" from the outer edge.