You start by heading all the way to the trailer park. Once you've arrived at the trailer park, head west. Stay behind the fences on your left when walking west until you stumble upon a computer.
Hit the computer until the screen turns blue. Then run away from it because it will explode not long after said screen turns blue.

This next part is probably the most complicated part. If you keep heading west after you blew up the computer you'll eventually come across Johnny's bloody head on a spike. You'll need to hit this head at a very specific time to spawn in the next step. I'f a message saying "Something has appeared on the map" fails to appear, then you'll have to start from step one again when the computer spawns back.
If the message does appear, then the next step is ready.

All you need to do for step three is to hurry back to the trailer park and shoot the 2 cows on top of the pink trailer in the center of the park. Shoot them until they explode.

Once you've done this then all you need to is walk back west for a little bit. Kevin should appear at your location shortly after you've walked enough.



Note: I had to keep retrying step 2 at random times to get this to work. I don't know the time frame your supposed to hit Johnny's head at.