To get this achievement you'll need to progress through the first bit of the mission where you're sneaking past/killing the enemies. After this run past Captain Price and run down the hill straight away. You'll want to be down at the bottom of the hill before he starts talking about the Predator Missile.
Once at the bottom you'll want to go prone next to the dumpster and wait for Price.
If you're still struggling, feel free to use this video made by BreakingBear101 as a reference.
Hope this helped.