Make sure to make a backup when you go outside to look at the fortifications. There may be some issues with getting this to pop, so it's good to be able to go back to the beginning of this scene.
This is for doing everything in the yard before you talk to Violet.
She'll come out, but as long as you don't talk to her, you should be able to finish looking around.
Some people have said talking to people interferes with the achievement. When I got the achievement, I talked to absolutely everyone, so talking to people does not interfere with this. The last thing I did was go to the graves, and it popped. You do need to talk to Rosie and Louis for achievements.
You probably don't need all of this, but here's a list of everything you can do in the yard. Talking to people probably isn't part of this, but it covers everything.
Look at the graffiti
Look at the flagpole
Look at the head next to the flagpole
Look at the walls (on the gate)
Look at the barbed wire on the walls (I can't remember exactly where this was)
Look at the pile of heads on the ground
Examine the wooden spear barricade
Flip over the picnic table
Look at the burned out building (rubble)
Look at all of the graves (Marlon, Brody, Ms. Martin, etc.)
Talk to AJ & Tenn up on the tower
Talk to Mitch
Make sure to pet Rosie for:
Shoot the bow with Louis. Don't miss for:
If it hasn't popped yet, make sure to look around one more time for anything you can do. Once you talk to Violet, you can't go back.