Trim: Slice top third off each with a serrated knife. Remove outer leaves until pale-green ones are exposed. Remove stems’ tough layers with a peeler; trim ends. Trim tough leaves around bases with a paring knife. Keep in a bowl of water with juice of 1 lemon.
Steam: Bring 2 inches of water to a simmer in a large pot fitted with a steamer basket. Add artichokes; cover and steam until easily pierced with the tip of a sharp knife, 10 to 12 minutes.