In this DROPS video we show how to add a tightening thread.
ometimes a toe or a hat or another design asks to tighten the edge together when assembled. You can sew in a tightening thread afterward but the easiest and with the nicest result is if you let a tightening thread follow the edge while you are working the piece. Insert a matching thread in the outer loop of st and let this thread follow the edge, make sure to wrap the working thread around it when turning at the edge.
Afterward pull hard, tie and fasten tails.