The instructions say:
(K2, P2) around. Work in K2-P2 ribbing for 15 rounds. Work 3 rounds of stockinette stitch (knit every round). At beginning of next round divide for heel.
OK, so you've done your 15 rounds of k2-p2.
Now you knit one row. Then knit another row. Then knit a third row.
Alrighty, now we're about to start the fourth row. The instructions say this row is where we have to divide up the stitches for the heel.
DIVIDE FOR HEEL:
Transfer stitches around so that there are 12 stitches on the first needle, and 6 stitches on each of the other two needles.
At this point, you have 8 stitches on each needle.
1) Slip 4 stitches from needle 2 over to needle 1. That gives you 12 stitches on needle 1. So far so good.
2) Needle 2 now has only 4 stitches on it. Slip 2 stitches from needle 3 onto needle 2. Now you have 6 stitches on needle 2.
3) You will notice that you now also have 6 stitches remaining on needle 3.
It may look weird, but it will all work out in the end. Have faith!
So now you just forget about needles 2 & 3 for the time being. Those are "instep needles" and you will come back to them later. At this point, you are only going to work on the 12 stitches on needle 1. These are known as your "heel flap" stitches.
The instructions say:
Work back and forth across the first needle ONLY as follows:
Row1: Slip 1, knit to end.
Row 2: Slip 1, purl to end.
Repeat these 2 rows for 12 rows total, then work row 1 once more. The heel will be 13 rows long.
Yes, OK. We have needle 1 and we're ready to go.
So, slip 1, and knit to the end of needle 1.
Congratulations! You have just completed row 1 of the "heel flap". This was the "right side" (RS) of the sock - the outside of the sock.
Now, you turn your work around so you're looking at the purlside of the stitches on needle 1. The purlside is the inside of the sock's "heel flap". This will be row 2. Slip 1, and then purl to the end.
That's row 2 completed! Hooray!
Now, turn the work around again. You're looking at the outside of the sock's heel flap. Slip one and knit to the end. That's row 3 completed.
Now, turn the work around again to the purlside - the inside of the heel flap. You're ready to start row 4....
And so forth until you reach row 13 as per the instructions.
I hope that helps make it a little clearer...