Instructions
Click any patch of soil outside of your Sim's house. Select "Plant Many..." followed by a crop. The type of crop does not matter. Plant at least 10 to 20 crops.
Ignore your crops for three or four in-game days. Bugs should begin to appear on your crops.
Click on an infested crop and select "Spray" to spray the crops with pesticide.
Continue to spray each crop with pesticide until your Sim becomes enveloped in a green mist, turning your Sim into a PlantSim.