While in Vavilov you'll need to collect 4 canisters to bring a tree back to life. Earliest opportunity to unlock this achievement is during the mission "March of the Flowers." Which is to collect the 3rd canister from the Cryo Labs. Near the start of the mission you'll enter a large room filled with green plants, if you scan the room you'll see "Mothers" sat amongst the plants. These "Mothers" will spawn the Sprouts so don't kill them just yet!
Once you've completed the battery puzzle and powered up the room the Mothers will wake up and start launching Sprouts into the air. When a Sprout comes into contact with a dead body it'll transform it into a Mutant. The key here is to stand near a dead body and wait for a Sprout to enter it. Soon as the Sprout enters the dead body quickly hit it with a melee weapon to kill the host. If killed before turning the achievement should pop.
You'll get several other opportunities to get this achievement while inside the Theatre and Pavlov.