The white wolf discovered a book that gave him a new insight about one of his favorite potions:

Snorting white myrtle petals that were pulverized with Igni - combined with the energizing effect of Tawny Owl - gives him more control over signs.

-> For as long as the potion lasts, he can use all alternative signs.

In a magical place, where an owl receded and no light was needed

The white wolf found a burnt tome scattered on the floor with a tawny owl on display
Depicting symbols of chemistry and herbs which preceded
Another drawing - showing a nose and a finger 
The familiar symbol of fire ?  and - what looks like tinder
He couldn't fully read what it had to say - but for "white petals" and "ashtray"
And so curious was he that day - our witcher
He looked in his pocket with an expression of linger
Try - if he may - with the white myrtle on his finger?

- Dandelion


Note that only the base rank of each alt sign is unlocked by default.
For example, you will be able to use Firestream Rank 1/3 without investing any points.
If you use Firestream a lot, you could invest in 3/3 to get:
 Stamina cost is reduced by 50%

Requirements
 
The white wolf needs to be a skilled connoisseur of potions:

• Metabolic Control (General Perk: maximum Toxicity increased by 25 points)
  An intensified Tawny Owl is not for a weak stomach.

• (optional) Refreshment (Alchemy skill: every potion heals)
  Usually, sorting something that has just been pulverized with fire is not a particularly pleasant thing to do.But Geralt is too juiced up on Tawny Owl to notice. And his fast tissue regeneration mitigates any harmful side effects.
• (optional) Side Effects (Alchemy skill: every potion randomly triggers the effect of another at no additional cost)
  Potions are not just about externally induced biochemistry. Geralt has learned to draw inward - feeling the potions and the reactions in his body and amplifying the signals that he favors. Most of the time anyway.

He also should have an ample supply of white myrtle petals - since every completed use of Tawny Owl removes 1 from his inventory
(if the skill requirements were met). But just in case you really don't feel like collecting more petals(although they are very common) having them in the inventory is not strictly required.

Motivation

Skill trees in RPGs benefit from having active combat skills that a player can look forward to and that have a noticeable impact on the power of the character.
On the flip side, the combat could miss out on key mechanics until many hours of gameplay.
This might be a good thing if the goal is to introduce complexity over time. 
But it has lower priority when the player is already familiar with the mechanics and doesn't want to miss them.
And even for new players, passive abilities that have a less obvious influence on game mechanics, have to take the back seat as the general motivation is to just unlock the active skills to make combat more complex.

The Witcher 3 amplifies this set of trade-offs because skills have to be equipped into a very limited number of slots. 
You might not want to focus on signs and experiment more with different builds. But missing out on alternative signs - even if not used as frequently in a different build - can be a hard potion to swallow.

A week ago, I was very glad to have found the Warden's Armor set. Which has the bonus of unlocking all alternative sign skills.
It comes as part of the highly recommended mod Shields or  (originally)  More Robes. The downside was that I had to wear this armor to use alternative signs - making other armor obsolete.

I hope to bring a more balanced and fun solution with this mod by tying alternative signs to a potion,  a resource, and (depending on which file you choose) 1 - 3 skill slots.

Compatibility

Works with everything, except Mastery Training and my previous mod Alternative Signs Unlocked.


---------
Please also have a look at my mod: 
School of the Mongoose