The Turned Lapped Seam is very clever and offers a lot of great features. It's very strong, as the seam is sewn three times. The raw seam allowances are hidden inside, unlike the Lapped Seam, so it can be used on all kinds of fabrics. The seam gives a neat finish which is identical on both sides so it is perfect for reversible garments. You can also layer two fabrics together to feature a different fabric on each side.
You do end up with four layers of fabric in the seam, so it is not ideal for very thick fabrics.
For a 1/2" seam, you will need 3/4" of seam allowance on both pieces of fabric. So you need 1.5 times your seam width for each seam allowance.
How to sew a Turned Lapped Seam - Tutorial
To begin, overlap your fabrics the width of your seam (in my case 1/2"). Both fabrics should be right side up.
Stitch down the middle of the overlap. My machine's foot is 1/2" wide, so I lined up my fabric edges on the foot and stitched straight down the middle.
If required, trim your seam allowances down so they are even.
Here's where the word 'turned' comes into the seam. Fold each side of your fabric over the seam allowance.
Your fabrics will now be wrong side up and the raw edges of the seam allowances will be tucked into the folds.
Here's how it looks from the side. (Fabric is now right side up)
And here's a diagram to show you what I mean. The red line is your stitching from ealier.
Now press your folds into the fabric.
Edge stitch along one side of the fold.
Flip it over and edge stitch the other fold.
Here's how the seam looks from both sides.