The Airport card is located in the top left corner of the main logic board, just under the second HD carrier.
It is held by two Philips screws and has two black antenna cables connected to it.
Slide out the card from the edge connector, place the new card near the old card and carefully pop off each antenna and place it in the corresponding location on the new card. The cables marked 1 and 3 are the ones to use the number 2 cable is not used.
Lastly, slide the new card back into the connector & secure it back down. One thing I do is apply a piece of Kapton tape (non conductive tape) on top of the antenna wires so nothing can short across them.