This achievement can be gained super organically just playing through the game!
However, if you are focused on getting this you will need to head to the west wing of the p.d. You will go through the observation room navigate towards past the safety deposit room, go left there will be a hallway and some stairs go to the right of the stairs and there will be a door. Before entering the door there is a green plant, pick it up. Now go inside the door you are in a safe area. In the back room of said safe room there is a red plant, pick that up.
Now go into your inventory click on one of the plants, combine it with the other plant. And Boom Achievement.
P.s. This is for Leon's playthrough.