When I taught Bobby "peekaboo," I marked and rewarded just for going under my legs. As a puppy, my sister and uncle liked to play "tunnel" with him and he would enthusiastically go through legs with a Kong in his mouth. Afterwards, I would hold a treat in front of his nose longer, luring him to stay for at least a second and I would mark and reward. Bobby is pretty food motivated like all Labs are so I just held a stinky treat in my fist as he nosed and sniffed it and walked with his head sticking out. Small steps at first and eventually, from the back of the lounge to the front.