Funky Little Chair has a video that perfectly explains how to show a box cushion with welt. I'm just following their instructions:
Top and bottom pieces: the pattern repeat on the Tandoori will allow me to cut both pieces right next to each other.
Boxing: For boxing I'm cutting a 40" x 5" piece, with the front centered. Then cutting two more pieces to sew on the side to fill in the full length needed. The boxing folds on the side to hide the ends of the zipper.
Zipper: I bought 27" zippers. I'm cutting two 3" high pieces, one over the other (the width doesn't let me cut them side by side. After seaming that will be a 5" strip, same size as boxing.
The Ottoman will be identical size.
Here's the fabric layout:
The cutting layout will be approximately like this. The zip and front will be centered to the fabric stripes pattern. The box sides will need to be adjusted to match the pattern to the piece they're joining up to. (NOTE I've changed the zip piece to be 30" long, to give just a little more room at each end.)
I've cut paper pattern pieces using pattern paper, and need to be careful the paper doesn't catch fire when I'm cutting with the hot knife!
Cutting:
Cut out pieces with hot knife and mark insides. Make sure to keep centers and to match stripes where pieces are joined (boxing panels and zipper piece). Use the hot knife or chalk to mark the centers on each side. On the boxing and zipper mark where the corners are expected to be (they may be a little off when sewn).
Assembling (using steps in Funky Little Chair video):
1. Sew the boxing panels together, matching stripes. Press.2. Sew 1" at the end of each zipper panel, then center and sew the zipper
3. Sew the welting to the first seat panel. Start at the center back, leave 2" extra for joining, and stitch around. When you're almost at a corner cut 3 little notches to help the welting curve. you get to a corner give it just a tiny bit of curve - not exactly 90 degrees.
4. When you get to the center back stop a few inches before. Clip each side of the welting so they go about 1" past center. Open each side and cut the cord so it reaches just to the center notch. Flatten the fabric and fold each side so the cord meets in the center and the two ends just go past the other side, then stitch over them.
5. Repeat on the other seat panel
When both seat panels are done, start attaching the boxing and zip panels:
6. Start at center front, sewing the welt to one seat panel. When you get to just before the corner cut a little notch where the corner will be. Now fold the box panel and cut a notch on the other side - this helps to ensure that the box panel is even on both sides. Pivot at the corner and continue to halfway point on the box side. Flip the fabric, start back and center front, and repeat to sew to halfway to the other box side.
7. Repeat with the zipper panel, starting at center back and rounding the corner then stitching just 1". Flip and repeat on the other side.
8. Stitch the ends of the box panel and zip panel together on each side.
9. Fold to make a pocket so the box panel partly covers the end of the zipper on each side, and stitch.
10. Open the zipper, then repeat with the other seat panel.
The cover is now ready for dacron wrap.