Put the car up on ramps and Pop off the bottom aero panel and then remove and install the horns as by the DIY. One thing I found that makes it easier to install the new horns is to use a longer bolt than the one that holds the single stock horn an bracket.
In addition there is now a dedicated ground lead for the stock horn, so, make 2 pigtails, one for power and one for ground. No relay is necessary as the stock circuit is already controlled by one.