The first possible place to mix potions is in Gohart, in the western foremost building opposite the blacksmith. If you take a right when you enter, you'll be in the the basement. There you will find an alchemy mixing station and you can experiment. A few quick examples can be found in this thread HERE. You will also get this if you wait to do "Elixir of Fate."
Note that if you increase your Alchemy skill to level 10 before getting this, you'll automatically learn every recipe and will not be able to unlock it! Start a new save file if you do this by accident.