For me, the plate just snapped back into place. Wasn't easy but with some delicate maneuvering the perforated edge eventually snapped back into it's original position. Observe how the antenna plate fits as it is removed, remember not to bend or tweak the plate and there should be little trouble although the process is not like the guide by any means. Ok the other design change in the Late 2012 model is the antenna connector is hidden under the case, so unlike as indicated in step 10 you can not see it let alone remove it until the Logic Board has been eased out an inch, so just lay the antenna plate upside down on top of the case while the rest of the screws are removed and the logic board is eased out some which will provide access to the connector. same deal on the return.
On the Late 2012 model the antenna connector is hidden under the case, so unlike as indicated in step 10 you can not see the connector and definitely not remove it until the Logic Board has been eased out an inch. Note how the cable makes a loop under the case as it is removed. Remember to reconnect the same way with the loop and connected before the logic board is pushed in all the way.