This finishing is very commonly seen on armholes and neckline. But i do find it hard to explain it to my students sometimes, just by words. So here’s the pictorial version of it, hope it’s helpful for you.
STEP 1: PREPARE FOR BIAS FACING APPLICATION
The above image shows the blue twill fabric with its right side facing up. The bias facing is in white, cut to 2.5cm (1″) width.
STEP 2: FIRST STITCH LINE
Position and pin the bias tape to curve such that their raw edges align along the curve.
Stitch at 0.5cm (1/4″) away from the curved edge.
This is what it looks like from the right side.
STEP 3: PRESS BIAS FACING UNDER
This is how it looks like on the wrong side.
STEP 4: SECOND STITCH LINE
Stitch the bias facing (with the right side facing up) very close to the ditch, as shown above.
The above is what it looks like on the right side.
This is what it looks like on the wrong side.
STEP 5: TRIM BIAS FACING
Trim away the excess bias tape. The above shows the wrong side of the project.
STEP 6: PRESS THE BIAS FACING UNDER
Press the bias facing to the wrong side of the project.
You would not be able to see the bias facing from the right side of the project, as above.
STEP 7: THIRD & FINAL STITCH LINE
Top stitch at 0.5cm (1/4″) away from the curved edge.
From the right side of the project, you can only see the final stitching.
From the wrong side of the project, you can see the second and third stitching.