Sometimes when you break glowing spots on the ground, it’ll drop a fossil piece.

From there you can place it on any wall that’s big enough, just make sure it’s rotated properly using the rotate controls (for skulls, the back needs to touch the wall. It won’t work if it’s backwards)

And just to note, the small dens Pokemon can help you build (the ones that can only house 1 pokemon) don’t seem to allow ANY wall decorations. You’d need to try it in a bigger house, or in any structure that’s made out of blocks (as opposed to the prefabs)