How do you play for the World Series? Qualify for the playoffs. How do you qualify for the playoffs? With elite depth and talent. Now, obviously the Cubs lost last year but there is no "just do this and you'll win the WS". Also, taking advice from the Sox on winning the world series is like taking financial advice from a guy who won the lottery.
How do you play for the World Series? It's simple. You already know the Cubs are good enough to make the playoffs. So you focus on making the Cubs a stronger PLAYOFF TEAM. This means you have to address your one glaring weakness in the playoffs last year--starting pitching.
They have enough depth at the position spots, but they lack depth in the starting rotation. All the Cubs had to do was add another top end starter to slot behind Arietta and ahead of Lester, who was awful in the playoffs last year and is essentially a #3 starter at this point. The playoffs are about pitching, so the Cubs should load up with another top end quality starter. That's how you prepare for a World Series run.
Heyward, Zobrist and Lackey may help you win more regular season games, but they won't get you into the WS like adding Cole Hamels, or Jordan Zimmerman at the trade deadline would have. Acquiring Greinke or Price during the offseason should have been a priority for the Cubs, but they inexplicably chose to throw money at new position players known for their defense instead. You better hope they wise up by the trade deadline, or it's ground hog day against the Mets in the NLCS again.