Upon completing the last main story mission, "Ain't Nowhere Safer," you will unlock a side mission for MJ entitled "Herbalism." This mission allows you to partner up with MJ, using the Grow House to sell weed throughout New Bordeaux.
Once you have this mission, head over to MJ's new location and speak with him about the operations. The game will more or less guide you in how growing and selling weed will work. First, you will need to use the workbench in the Grow House to plant a strain (a type of herb). After this, you will need to wait a few (or several) in-game hours until the herb is fully matured and ready to sell to one of the nine districts in the city. While the herb is growing, you can check on its progress by opening up the world map and scrolling over the Herbalism mission icon. Here, you can take note of the "Growing Progress" stat to help you keep track of when to return to the Grow House.
When a strain is fully matured and ready to sell, interact with the workbench and you should notice a progress bar at the top of the workbench menu that indicates how much more money you need to make in order to reach the next Herbalism level. All in all, reaching the Max Herbalism level will require you to sell $50,000 worth of herbs.
Each time you hit a new Herbalism level, MJ will task you with roaming the city in order to find new strains that will yield better quality. At any time, you can use the herbs you have collected to create your own experimental strains, which often yield better results and sell for more money than their standard counterparts. In the end, it'll take a few hours to reach the Max Herbalism level, but it's not too difficult to get done and you can idle your character just outside the Grow House when a herb is growing to prevent a long drive back from the city.
NOTE: To help speed up the process of your strain maturing, make sure to return to the Grow House and Workbench and "Fertilize" your crop once it reaches the "Flowering" status. This will increase the rate at which it grows. Thanks for the tip, Aggro Graf.