Needles: US10/6.0 mm
Yarn: Gjestal Østlandsgarn (DK / 8 ply (11 wpi) 100% wool)
Woman: 19 sts, knit garter stitch 38 rows pr square
Man: 22 sts, knit garter stitch 44 rows pr square
Child: 14 sts, knit garter stitch 28 rows pr square
Baby: 10 sts, knit garter stitch 20 rows pr square
Knit squares, alternating colors. See diagram. The easiest way is to knit a contiuous strip of the first 6 squares, alternating colors. Then pick up and knit the stitches for the "flaps" at the sides of the strip.
Sew the slipper by joining matching colors from the diagram, red against red, blue against blue etc.
Felt in washing machine, 60-90°C. Put a pair of jeans in the washer with the slippers to felt them more.
I've recieved some comments that the colors of the diagram are hard to separate when printed, so I've added another diagram with letters instead of colors for people without color printers.