[Obsessive Costuming Dude (Alex)]

The curve is because the trousers (like the human waist) are slightly higher in the back than the front.

Your waistband facing assembly looks correct, although the orientation should be with the open/free ends flush with the center front of the trousers, and the center seam flush with the center back of the trousers.

Basically, wrap around around the top edge of the trousers, right sides sides together and matching the seam lines (side front, side, side back, center back).

Sew the facing assembly to the trousers along the top edge.

Then, in separate stitches, sew the front edges of the trousers/facing assembly together.

The only visual references I have handy are those already in the tutorial, but I did label the attached image to hopefully clarify for you. 🙂

– Alex