I already knit a pair of those for my mum years ago - you cast on 8 stitches less then you normally would for your size, then knit for about half the foot length plus 2 cm in garter rows, join for the rows up to when you would normally start the toe (plus 2 cm). In the end, you sew the heel as if it was a T - first 2/3 paralell, then both sides of the “top bar”.