It's really simple - all you need is the double crochet stitch
You will need:
Double knitting yarn in your favourite colour
4mm crochet hook
Co-ordinating fabric to line
Leave a long tail at the start - make your starting loop
Foundation row: chain 37 loops.
Row 1: starting in the 2th chain from your hook – work 1 double crochet in each of the chain stitches.
Row 2: work 1 chain, work 1 double crochet in each of the double crochet.
Turn the work after every row so that you are working back along your previous row.
Repeat until rows until your work measures 8” / 20cm. Finish off leaving a long tail.
fold the pencil case and sew up the sides - catching just the very edges of the crochet.
I get my zips by length - it's much more economical that buying a 'finished' one - and you can always get the size you need.
Sew the crochet to the zip closing in the base of the zip.
It is fine for the zip to sit proud of the top edge - it has become a design detail for the zip to show now. You can even get them with patterned edges now