I started this process after finishing the first 2 missions. After that time there are soldiers wanderings the streets for you to consume saving the other achievement where you have to finish the game consuming 10 or fewer citizens. Just start consuming soldiers by grabbing them with B and hitting Y for the consume. If you happen to grab a citizen hit B again to throw them saving the secondary achievement. Also if you can't find soldiers for some reason just start hurling cars all over the place and this generally will attract enough attention to bring in soldiers.