crochet 4 mm
sc = single crochet
Crochet 15 ch (or as long as you need it).
sc in every ch
make 3 sc in the last chain and go on crocheting under the chain (see diagram)
make 2 sc in the last chain.
Work in continous spirals until you reach the desired heigh. Change colours as you like.
Crochet the last round in back loop only. Leave a long tail to sew the zipper.
Step 2: The Strings
I've also used a clip to attach the coffee machine key, so I can untie this key even when I'm using the USB key connected to the PC.