I Googled for these and found that they seem to be called Mary Jane slippers, not sure if that's just what they call them in the US though. Anyhow I have found these instructions for you, hopefully you can adapt these if necessary.
How I made them:
I used chunky yarn and size 7 US needles.
-cast on 30 stitches
-knit for 7.5-8 inches in garter stitch -- eyeball it based on the length of your own foot, I wear a size 9-10 US
-k2tog for one row
-k2tog for another row
-cut off a remainder of yarn, and thread it through the stitches like a drawstring.
-fold your fabric in half and, using the remainder yarn, sew the edges together for about 3 inches.
-using the tail of your working yarn, sew up the vertical seam for the heel
To make the strap, I crocheted a double chain with a loop at one end. I sewed one end to the slipper, and sewed a button to the other side.