One of the first locations you can visit in the game is the Ag Centre. Here, you need to get infected by the exploding plants (which is likely to happen without effort). Check your character stats for a green square to make sure.
Now, for the achievement you need the infection to worsen and finally turn the character into a pod person. This will take about six hours of real time. So make a separate save file, and plug your controller in so it doesn't turn itself off, and leave your game running while you're away.