Lin Mu listened to the Saintess' words in detail and memorized them. Since he already had a decent mastery with the formations, it wasn't that hard for him to fully understand the methodology.
The method was simple for him, as it was the same as creating small runes that could be linked together into a tiny formation that would be able to work on the veins.
Of course, making formations on living things was entirely different than laying them on inanimate objects. Not only were there greater chances of making mistakes, there were also factors such as the natural flow of Qi and vitality that could also affect the formations.
Either they would become disturbed, or would not be able to be created. And even if one did manage to make them, there was no guarantee it would be stable enough to work.
As such, one needed to be an expert at formations in order to make them work on living beings.