How to plan an ambush
First ya find a good place to have an ambush. Ease of access and egress. Ability to get in and out without being detected. Vantage point with ability to see targets well while remaining concealed. Large kill zone. Nowhere for targets to hide. Then ya find a way to lure your target(s) into it. That's what I believe happened in Dallas. The snipers did not just happen to be there when the cops walked by. They knew where the cops were going to be ahead of time. Maybe even months ahead of time. Maybe they practiced several times. They were just waiting for the next Peaceful Protest to spring their trap.

Liberals are like a "Slinky". Totally useless, but somehow ya can't help but smile when you see one tumble down a flight of stairs!