After reaching Skyhold and completing its 1st set of repairs (simply leave Skyhold and come back), you can enter the garden through the door in front of Varric's default location. At the beginning, you have two garden pots available by default. After reaching Skyhold, you now have a random chance to receive seeds for the plants that you harvest. Keep harvesting plants, and you will eventually get their seeds.
After getting the seeds, go to the Skyhold garden and plant any of the seeds in the garden pots. Their growth have three stages. Planted, growing, and ready to harvest stages. They will be at the planted stage first. Moving to another location and coming back to Skyhold moves them to the next stage. Keep moving and coming back until the planted herbs are ready to be harvested. After harvesting, they go back to the growing stage. So you can repeat this process to easily farm 50 herbs and get this achievement. However, if you plant a different seed, this process will start over from the planted stage.
There is an option to upgrade your Skyhold garden to an herb garden, which has a higher number of garden pots. To unlock this, first go to the Inquisitor's Quarters (through the door next to the Inquisitor's throne). In this room, on the table, you will find a note.
Credit to FelixDracul for their
guide.