Reading and understanding the recipe booklet
When you begin the process of Alchemy in Kingdom Come: Deliverance 2, you always need to look at the recipe booklet. This shows you the required reagents, their respective amounts, and the steps that you need to follow with absolute precision.
Likewise, if you have the ingredients in your inventory, you can just hold the “Prepare” button prompt. This will automatically cause the reagents to appear in neatly placed jars that you can interact with, instead of having to select them individually from your inventory.
Lastly, note that some locations with Alchemy benches are quite dark. It’s best to bring out your torch before starting the process. This allows you to glance at the booklet so you can reread the instructions.