This is an amazing plant!!!
I wrote an article about using nettles, and lamb's quarters can be prepared and harvested much the same way. You can even nip off the terminal buds and eat them like with nettles, and then the lamb's quarters will send out multiple terminal buds to eat later in the season. The nettle article is here: http://yostsurvivalskills.com/wild-edible-plants-2/
I usually wait to harvest any of the plant until it is abotu 12" high and has a good start, because if you pick the shoots, you car really set the plant back.