I've gotten a couple questions through the Sweet Mary Janes KAL, concerning the heel, So, I thought I would attempt to walk you through how I do it.
My slipper. I have just knit my last row, and I am about to cast on more stitches for the heel.
I do not turn, instead I cast on the number of stitches that pattern requires.
Then I rearrange my stitches on three needles, (I prefer to use three needles. Feel free to use four.)by slipping them to the other double points, as of I were purling them.
Now, make sure your stitches are divided into equal numbers for top and bottom. You'll have to knit the first two stitches of the next double point to do this. This will mean the you'll have two stitches on each side of the top half the were part of the section you knit back and forth on previously.
Now you will knit one round, and continue on as directed, decreasing like one would for a sock toe.
I hope this is helpful.