This was done on a 2001 XLT 4.0 auto with a tranny cooler that has been body lifted, so your mileage may vary if this will work for you. But it should for most applications.
First step, remove the grille and the air damn behind it. Not hard, a few screws, some plastic retainers, and then some metal clips. Great opportunity to paint your grille insert if it's faded like mine was. I did. Remove the factory horns. Throw them in the trash where they belong.
Next, make some brackets.
I bought some aluminum stock from Home Depot for cheap.
It was 36 inches long (you'll only need 20), 1 inch wide, and 1/8th inch thick. You could buy steel, which is stronger, but it's also heavier, and will rust if your paint flakes off.
Cut 2 pieces 10 inches long. And drill 2 holes in the center of each, both holes 1 inch from the edge. One 5/16ths (where the horn goes), the other 3/16ths (where the bracket will be bolted to the core support).
Paint said bracket. I used some duplicolor spray on bedliner, because I had a few cans laying around, also the same stuff I painted my grill with.
I also took the time to remove the grills from the horns to paint them ford blue (my truck is blue, and I figured it would match better than the stock red grills). Make sure you only remove the phillips screw and not the big nut on the back that holds the bracket on, otherwise the horn probably won't ever work right ever again.
Mount the horn to the core support using the bracket you made. There's an 8mm bolt right at the top of the headlight bucket. Take bolt out, put bracket over hole, reinstall bolt. Use some 5/16ths bolts, washers and nuts to secure the horn to the bracket.
Wiring. The kit comes with a relay, you can use it if you want. I didn't. My stock horns died, I was replacing them, the factory horn relay is the same amperage as the one the kit comes with. I also gave 2 shits less about the factory horn connector, I chopped it off since my stock horns were dead.
You'll need to make 2 identical wiring harnesses like this using female spade connectors. Basically, we are wiring the horns together in series, just like the factory horns were. So you'll have 2 wires going into one spade connector. I didn't measure this out. Sorry, but it's easy to figure out.
Connect the 2 harnesses you made to the factory horn wires. The blue is positive, the black is ground. Does it matter which part of the horn you hook up to? No, it doesn't. I hooked the blue wires up to the top connector on both horns, and the black to the bottom wire. Just match them, and even if you don't, it won't matter. I used heat shrink butt connectors to make my connection. Ideally I would have soldered and heat shrinked it, but I was getting a bit drunk at this point, and couldn't find the right size heat shrink in my garage. Whatever, they both work fine.
Zip tie up any loose wires, you slob.
You should look like this now.
Reinstall your grille.
LOOK!!!! TITTIES!!!
And enjoy.