To prepare herbal extracts in The Witcher 3, you must first advance along the storyline to a certain task. Now you can make out the path to this mission step by step, starting from Vronice - the possession of the Bloody Baron.
Story walkthrough
Long before preparing herbal extracts in The Witcher 3, Ger alt begins his journey to alchemical experiments with Kaer Morhen. This witcher fortress is located in the very north and is considered a stronghold for the witchers of the School of the Wolf.
After he and Yennefer raised one of the inhabitants of Skellige, Skjal, from the dead, Ger alt learned that the path to finding Ciri lies through the freak. The most disgusting creature, over which a heavy curse weighed, was located on the territory of the castle of Vronitsa.

The Witcher goes to Velen, from where he takes Uma and continues on his way to the castle of Kaer Morhen. On the way to the northintercepted by the envoys of Emperor Emhyr and demanded to bow to the ruler of Nilfgaard.
Main Quest
Ger alt's journey continues, and after a while he gets to the School of the Wolf. On the spot, he is met by a disgruntled Vesemir, who complains about the black-haired sorceress, after which he asks the witcher to go after Eskel, who went hunting for the forktail.
Further, when the forktail is killed, the protagonist returns to the castle and finds Yen making an unsuccessful attempt to activate the megascope. It turns out that it does not work due to interference, which Ger alt will have to identify. To do this, the sorceress gives him a potestiquisitor - it helps to identify the cause of the interference. On the lower floor near Vesemir, you can find a box with dimerite, which was causing interference.
When the problems are fixed, you need to activate the megascope with Yennefer, thanks to which they will contact Ida Emean, and she will tell about the essence of the curse and how to deal with it.
After this conversation, the sorceress has an interesting idea, why you need to prepare herbal extracts in The Witcher 3.
The Enchantress's Decision
Before removing the curse from Uma, you need to prepare herbal extracts in The Witcher 3. However, reaching this decision will not be easy. First you need to infuse the phylactery with elemental power.
During the conversation between Vesemir and Ger alt, the witcher finds out that there is one way to deprive the curse of power, but for this Vesemir had to go with Uma to the top of the mountain and perform a small ritual. So the witchers got a freeevening.
Here it depends on the player how the main character will spend it - together with the sorceress or with old friends. One way or another, everyone will learn about the option from the sorceress.
Yennefer decided that the curse should be removed gradually, and it should start with a test of herbs. The witchers expressed their dissatisfaction with this, because then the freak would experience incredibly severe pain. So you need to prepare herbal extracts in The Witcher 3 for this. The passage of this mission does not require important decisions, so there will be no consequences.
Witcher Mutations
Witchers are tested by herbs in their teens, and only one in ten boys could pass this test. This procedure, which requires the preparation of an herbal extract in The Witcher 3, opened the human body for further mutations.
This is exactly what Ian wants to do - to open up Uma's body for further changes. After that, she will ask Ger alt to prepare herbal extracts in The Witcher 3.
Finishing the potion
So, morning came, and Vesemir's campaign did not give significant results. It's time to use the sorceress option, for which you need a guide to herbal extracts in The Witcher 3.

When all the preparations are completed, Vesemir and the sorceress will ask you to create the desired potion, and the ingredients are on the table that stands nearby. You can take absolutely everything. What Ger alt does not use in the task, he can keep in his inventory. Now you need to go to the Alchemy menu and selectitem "Quests", where the desired potion will be displayed.
Next, you need to go to the test table and wait for Yennefer's command. She will ask the witcher to introduce the extracts one by one, and the order does not matter. One thing is clear - Uma will be in terrible pain with each pitch.
After the introduction of all the extracts, the sorceress casts a spell, but it does not work, and Uma ceases to show signs of life. Then Yennefer, out of desperation, begins to violently beat the body of the damned, thus trying to bring him to his senses.

Her efforts are not in vain - after a few moments, Uma tries to pronounce the words in the Elvish language in a voice that is not her own, and Yennefer picks them up, completing the spell. After that, Ger alt opens the phylactery, and the curse on the creature is removed.

As it turned out, this creature was Avallac'h - a mysterious elf who accompanied the Swallow on her travels between worlds. He tells everyone that Ciri is on the Isle of Mists and needs help, as it is not safe there. The elven sorcerer cannot accompany Ger alt, and the witcher goes in search of his protégé.
Thus ends one mission and begins another, which will advance the player further in the story.