Video Guide!
This is completed during The Wasteland Courier side quest which can be started by meeting the merchant Daiko in Thirdborn. Additionally, make sure to bring Kai as one of our companions as he seems to be the trigger for this achievement to happen. After speaking with Daiko to start the quest we need to find some graffiti nearby on the Dead Swordfish shop. This will direct us to our next objective.
From Thirdborn, we want to head north to Deadfall Highlands to find a red flag with an X on it on a nearby cliff. Then we just need to make our way up dealing with some enemies along the way before meeting Aiko.
In this conversation we just want to be friendly and choose options to get the totally legal substance from Aiko. Then after he dies at the end of conversation we can leave the camp. As we exit, our companion will interrupt and bring up the idea of trying the stuff ourselves. Make sure to agree to try it to pop the achievement