When you know you'll be washing an item you sew, you'll want to make sure that the seams are finished to prevent fraying. If your fabric is sheer, you may also want to finish the seam in such a way as to prevent the ugly cut edge from showing through. There's lots of ways to do this, but in this instructable I'll show you how to finish your work with a French seam.
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
Place your fabric pieces together with the WRONG SIDES facing in. This is different from how most things are sewn; you're probably used to putting the right sides together so that the seam is on the inside. Please ignore the chattering squirrels in your head telling you that you're doing it wrong. Put the wrong sides together, right sides facing out.
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
Press your seam open. Trim 3/8" off of both seam allowances, leaving only 1/8" on each edge.
Step 3: Right Sides Together!
Now you can finally sew the fabric with the right sides together so that the nattering squirrels in your head will shut up!
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.
Step 4: Done!
You're done! You've got a nice seam that encases the funky raw edges within itself, and the squirrels in your head are at peace.