The Stock Pile is the bottom left pile of cards. Start a new game of Pyramid. Look at the face up card on the Stock Pile and see if it can be paired up with any of the seven cards at the bottom of the Pyramid. Draw that card and see if the next card can be paired up. When you have found three in a row that make pairs, use the Undo to put the first two back on the Stock Pile, then use them to remove the three cards from the Pyramid.
Do the same thing, only with three cards from the Waste Pile (bottom right pile of cards) to get the achievement