Along one long edge, press under 1cm to the wrong side twice. Edge stitch in place. Do the same on the other back panel piece but ensure you make this seam on the opposite side. The two back pieces should be a mirror image of each other and the hem you’ve just created should be at the top.
Fold over the top of the panel by 1cm twice and press, do the same with the other back panel.
To angle the strap pieces: Lay out the strap pieces above the back panels and cross them over each other.
Measure up 2.5cm on the inside edges, make a mark and draw a line from this mark to the opposite corner. Snip the small triangle off and discard.
Tuck the angled raw edge of the strap into the top hem of the back panel- tuck it so that it sits approx. 0.5cm away from the side edge. Pin and stitch the top back edge.
Fold up the strap at the seam and pin in place.
Sew the strap to the top edge of the back top hem – use a longer stitch as there will be many layers to sew through. You should end up with a mirrored pair of strap/ backs.