Size 7 needles with 4 ply worsted yarn (or thereabouts – they invite experimenting)
This is a variation on the basic pattern for pocketbook slippers: cast on 5 stitches, knit one row, purl one row for six rows ; then, keeping in stockingette stitch, increase in the second stitch of each row until there are 22 stitches. On next row increase in each stitch so there are now 44 stitches. Change to moss stitch (K1 P1 across first row P1 K1 across second row) for six inches.
On the next row K2 tog in each stitch (22 sts) and now decrease by knitting (or purling; stockingette stitch again)) the second and third stitch together at the beginning of every row until there are 5 stitches. Continue in stockingette stitch for the next six rows. Cast off. Fold in half and join the moss stitch sides together and then join the two 5 sts ends to form the strap. I crocheted the buttons and sewed them on; they are purely decorative – the slippers are stretchy and slip on and off easily. Google images for an abundance of photos of sideviews and other variations.
Thay make up quickly and you can do a pair at once on the same needles. Nice for gifts.