Strange Horticulture

On day 10 you'll receive a visitor who's returning your brewing equipment to you. This will allow you to start creating elixirs from a combination of three different plants. You'll also receive a letter detailing Baylock's Elixir, but rather than telling you the specific plants to combine, it includes sketches of their flowers instead. By day 11, you'll also receive a visit from Amos Duncan who requests this elixir.

If you've been collecting all the plants from visitors, clues and the map up to this point of the story so far, then you should have the 3 required plants by day 10, and certainly by day 11. However, you may not have identified all of them. The required plants are Eyebright, Palliance, and Prittle (I had only identified Eyebright by day 11). If you study the sketches carefully though, it shouldn't be too difficult to pick the right plants. Eyebright has 9 red petals with a yellow flower center. Palliance has 5 light blue petals and two stamen. And Prittle has 5 light purple petals with two stamen. I've added a screenshot below which shows the correct plants (will add to guide once my Xbox captures sync to TA).

As soon as you brew the elixir the achievement will unlock.