Slipping the first stitch makes a nice clean edge. Just do this on every single row and you'll see the pattern emerge.
(If the edge stitches are throwing you then just take them out. CO your original 21 and k2p1 only on both sides.)
Here's my scrunchable done just the way I said earlier sl1 and k the last. I used Encore Worsted.
YOU CAN DO THIS!