There are a few things to consider before trying for this achievement. First, it is most easily achieved early in the game before every base is covered in infected detectors. Second, this achievement should be done simultaneously with the Brain Trust achievement which is to complete all consume events because many of the military consume events are infiltrating a base. With that said each base is marked in blue on your map, but when you are doing a consume event it will be marked in green as well. Go to a base disguised as a military personell and find the commander. He wears desert camis and has a cap and shades on. Stealth consume him so you don't alert anyone and enter the base by approaching the yellow cone of light and hitting Y. When inside of the base there will be one guy with a green double helix above his head. Stealth consume him. I would advise stealth consuming all of the guys with blue double helices as well because they upgrade different abilities, such as machine gun, rocket launcher, etc.. and those upgrades are needed for the Unnatural Selection achievment. After you are done consuming all important individuals, return to the door you entered through marked by the yellow cone of light and hit Y again.