If you have already completed Sozo's missions in the Spore Grotto then you can skip this step. However if this is your first playthrough then you will need to meet Sozo in the dungeon which will unlock the Spore Grotto location for you to visit. After unlocking this area you will have to complete some quests for Sozo:

• Give Sozo 10 mushrooms
  
• Give Sozo 20 mushrooms
  
• Preform the Brainwashing Ritual
  
• Build the Mushroom Sculpture

If you are a previous player this is where you will start working for the achievement.

Once you have finished all the missions for Sozo and return to the Spore Grotto you will find poor Sozo has passed away in your absence, however you can collect the red mushroom from atop his head. Go back to your cult and plant this mushroom in a farm space and wait until it grows (The Ritual of the Harvest will make it grow instantly).

Once the mushroom has grown you can harvest it to reveal Sozo as a new follower, they will instantly ask you for 5 mushrooms, once you give them these mushrooms they will run around your cult eating any other mushrooms they can find or just wondering in a dazed state making some silly sounds (They will not work or contribute to your cult in any meaningful way).

After a day Sozo will ask you to find there lost Mushroomo in the Anura, if you accept this mission and complete it Sozo will eat the poor little Mushroomo before giving you the same quest the next day. If you refuse the quest Sozo will become a dissenting follower and you must lock them in prison.

After re-educating Sozo they will transform into Dr Sozonius and after some short dialog the achievement will pop.

I hope this helps <3 <3 <3