Very powerful product!
I would start with a small amount and add a little more if needed.
Personally, because it is quite strong, I dissolve some quantity of the shampoo in a bowl with lukewarm to warm water and use it that way.
By slowly pouring it onto the roots, it runs through the entire length of the hair. This way, the color is distributed everywhere and discoloration is avoided!
After I have poured it everywhere, I give a gentle massage to the scalp and it creates a nice rich foam!
ALWAYS use a mask/conditioner/treatment after washing because, like with any silver/purple/blue shampoo, you notice a slight dryness in the hair.
I recommend it!! It is one of the best out there to keep away yellow/orange tones.