Really easy and fast to make.
Mods I made:

replaced the first 4 rows in garter st by
* JMCO twice the number of stitches onto
2 circular needles,
* work 6 rows stockinette on half the
stitches on 1 circ. need.
* folded work in 2 and pass each st of one
needle over each stitch of the other

Then I followed the pattern but worked in the round.
Before knitting the sole in garter st I added an inside sole i had knitted previously by passing each st of the sole over each st of the slipper.

I finished knitting my outside sole in the round decreasing 4 st on each even row.
Closed the last stitches with kitchener st after 10 rounds.

I had reversed the instruction to make the inside sole.

Finishing with a 3st/102rows i-cord to close the slipper, make a nice knot and hold it tight on my foot.