First, buy the "Serpent's Glint" (green) skill to unlock the Flare Round attachment for the pistol. Then, fast travel to the "Mission Of San Juan", Base Camp "Ruined Tower" region. There are four turkeys running around the base camp in this area. Search around the ruined tower, as you may have to walk around a little bit before they spawn (they are usually sitting in the bushes and start running when you go near). Shoot a turkey with the pistol's flare round to get the "Thanksgiving" achievement. Note: When you acquire this skill, you automatically get a few flare rounds, but they can also be crafted. You can change your save slot before doing this and reload it to get the skill points back.