Hi crochetknitfan,
I would continue to go in the back loops of the stitch when you decrease so it looks the same as when you were doing a sc in the back loop of the stitch.

The section or part that you just crocheted they want you to fold in half. After you fold it in half then match up the stitches on the inside of the sides you folded(the inside loops) and ss them together.

If you still don't quite understand what they want then post a picture of the slipper sock you made so far. I can't guarantee that I'll be able to figure out what they want you to fold in half but someone might be able to help you. I will try to help you though if you need more help.