This assumes you already have a bias strip. I used a 1.5" strip here. For outer curve, pin bias strip so the fabric is looser. Bias strip will be covering less distance. Pin raw edges together to right side of fabric.
After stitching, press seam allowances towards bias strip. I used a 1/4" seam allowance.
Now fold bias strip over the seam allowances and press. Here is an image from the back where you can see the raw edge of the bias strip.
Here is an image from the front where you can see the pins. Now stitch in the ditch.
For the straight side, just align the straight edges of the fabric with the bias strip. One doesn't need to be looser than other.
If your stitch in the ditch stitches are visible, wet your finger and roll the binding over them and then press.
The finished seams. (Yeah, I know the picture is a little fuzzy.)