The following steps are for all variations. Today we will add the yoke, sew the seat seam, attach the gusset, attach the lower back legs, and apply the hem reinforcements. As I sew I will also be demonstrating how to do flat felled seams on the right side of the fabric vs the wrong side for a true flat fell seam!
ATTACH BACK YOKE
With the wrong sides together, match one back yoke piece to the top of the corresponding back piece. The wider end of the yoke will be towards the rise side and the short end of the yoke will go towards the side seam side.
Align notches and pin them together.
This part is very important for utilizing your topstitching thread correctly. Before sewing, position your pieces so that the back pant is facing right side up. Thread the needle side of your machine with topstitching thread and keep regular thread in your bobbin.
Stitch along the seam with a 5/8" (1.5 cm) seam allowance.
Press the seam allowances open along the stitch line.
Trim the seam allowance edge of the BACK YOKE only to about 1/4" (6 mm). DO NOT TRIM THE BACK PANT SEAM ALLOWANCE. We will use this to create the fell.
Fold and press the seam allowance edge of the back pant to be about 3/8" (9 mm) wide.
Press the folded-back pant seam allowance up towards the back yoke and cover the trimmed back yoke seam allowance edge. Trim away any loose fabric threads and make sure no raw edges of the seam allowance are showing.
Using topstitching thread, edgestitch along the folded edge of the back pant seam allowance, through all layers, to complete the flat felled seam!
Repeat for the other back yoke and back pant pieces.
SEW BACK SEAT SEAM
Now I'll show you how to do a mock-fell seam finish when sewing the seat seam.
With right sides together match the back units along the seat seam. For a professional look, make sure the back yoke seams and topstitching are aligned.
Using either topstitching thread or regular thread, sew the seat seam at 5/8" (1.5 cm).
Finish the seam allowances together using your preferred method (zig-zag, serge, binding, etc.) and then press towards the LEFT back.
Flip your back pieces so that the fabric is right side up.
Using topstitching thread, edgestitch, and topstitch along the seat seam.
This may be another great time to use your seam jumper when sewing over the yoke flat-felled seams. Because this intersection can be a bit bulky, sometimes the foot and needle will get caught on the seams causing wiggly stitches or tiny, close-together stitches.
Open the inseam of the back legs so they are as flat as possible. You will see there is a slight peak where the two back pieces meet at the crotch.
With the right sides together, match the center of the gusset with the back crotch seam and pin.
Ease the edges of the gusset and back legs to match. It may feel a little awkward because you are matching two different curves together. The fabric may bubble in some spots but that is ok!
When sewing, the fabric only needs to be smooth at 5/8" (1.5 cm). Push any bubbles away from the stitch line as you are sewing. Everything will lay nicely once it is pressed.
Finish the seam in your preferred method with the seam allowances going towards the gusset.
ATTACH LOWER BACK
With the right sides together and notches aligned, match the lower back leg piece to the back. The gusset seam will align with the notch closest to the inseam on the lower back.
Finish the seam with your preferred method with the seam allowances going towards the lower back leg.
VARIATION 1 ONLY: ATTACH BACK HEM REINFORCEMENT
Fold the top edge of the back hem reinforcement pieces to the wrong side at 5/8" (1.5 cm) and press.
Match the back hem reinforcement to the hem area of the lower back, aligning the folded edge of the back hem reinforcement to the notch on the lower back leg.
Using topstitching thread, edgestitch along the top folded edge, through all layers. Make another row of stitching 1/4" (6 mm) away from the first row. Baste around the other edges to keep the hem reinforcement in place.