Abbreviations

k : knit
k2tog : knit 2 together
p : purl
p2tog : purl 2 together
pm : put marker
RS : right side of the work
sl1 : slip stitch with yarn in front
sm : slip marker
tbl : through back loop
WS : wrong side
yo : when working the yo, purl them through the back loop

PATTERN

Cast on 3 sts.

Row 1: (k, p) in the first stitch, p, (k, p) in the last stitch - 5 sts
Row 2: sl1, p, yo, k, yo, p, k - 7 sts
Row 3: sl1, k, p tbl, yo, p, yo, p tbl, k, k - 9 sts
Row 4: sl1, p, k, p tbl, yo, k, yo, p tbl, k, p, k - 11 sts
Row 5: sl1, k the k, p the p up to the yo, p tbl, yo, p, yo, p tbl, k the k, p the p

RS: sl1, k the k, p the p up to the yo, p tbl, yo, k, yo, p tbl, k the k, p the p
WS: sl1, k the k, p the p up to the yo, p tbl, yo, p, yo, p tbl, k the k, p the p

Continue to work these increases until you have 65 sts, ending with a WS.

Now, stop increasing. We will now work on the 17 central sts and there will be 15 between the markers.

RS: sl1 wyif, k and p the sts until you have knitted 7 sts after the middle stitch (WIHTOUT INCREASING). Place a marker. K2tog tbl and turn.

WS: sl1 wyif, sm, (k, p) 7 times, k, place a marker, p2tog. Turn.

RS: sl1 as if to knit, (p, k) 7 times, p, k2tog tbl. Turn.
WS: sl1 wyif, (k, p) 7 times, k, k2tog tbl. Turn.

Repeat those two rows (ending on a WS) until all the stitches are used and 17 sts are left on the needle.

Sides

With WS facing, lift 13 sts purlwise, from the side of the slipper, turn

If you wish, on the RS, you can slip (as if to knit) the 2 stitches along the sole of the slipper. The instructions indicate to knit them but you can slip them to make a nice definition of the sole.

RS: sl1, k5, pm, (p, k) 12 times, lift 13 sts knitwise, from the side of the slipper, turn
WS: sl1, k5, pm, (k, p) 15 times, k, sm, k6, turn

RS: sl1 wyif, k5, sm, (p, k) 15 times, p, sm, k6, turn
WS: sl1 wyif, k5, sm, (k, p) 15 times, k, sm, k6, turn

Repeat those 2 rows until desired length.

I did 7 ridges before starting the back of the sole (for my feet US 8).

Back of the sole

The back of the sole will be worked on the 9 central stitches.

13+17+13=43 sts (actual nb of sts on the needles)
43-11=32/2=16 sts

RS: sl1, k5, sm, (p, k) 5 times, k2tog, (p, k) 3 times, p, k2tog tbl, slip one stitch from left to right needle, turn
WS: p2tog, (k, p) 3 times, k, p2tog, slip one stitch from left to right needle, turn

RS: k2tog, (p, k) 3 times, p, k2tog tbl, slip one stitch from left to right needle, turn
WS: p2tog, (k, p) 3 times, k, p2tog, slip one stitch from left to right needle, turn

Repeat those 2 rows until all the sts between the markers have been knitted.

RS: k2tog, k7, k2tog tbl, slip one stitch from left to right needle, turn
WS: p2tog, k7, p2tog, slip one stitch from left to right needle, turn

Repeat those 2 rows until you have 9 sts total on your needles, ending on a WS.

Bind off: sl1, k1, psso, (reverse yarn over, k1, psso) repeat until the end.

Original pattern from a Russian web site: http://www.liveinternet.ru/users/lidav5/post209595188/