Then Kathy, aka grumperina, enlightened me on this. I started to understand why I couldn't see the "how" myself at the first place: I don't like grafting! So, it was being mentally blocked away. :) This perhaps will explain to you why I don't like grafting (and please just ignore the ladder!):
It is usually okay when I just have to graft, say, less than 10 stitches. When it goes beyond this number, things will become, literally, ugly. This is the first sock I completed ... which means I still have the chance to do better with the second sock. Knowing my own weakness, of course I would not be as bold as Kathy by grafting on the instep side of the sock:
Ugliness had better be hidden. Where can you find a better place to hide than by stepping on it?