To reach a new upgrade level, you will need from two to six Scrolls. A Roman numeral in the name of the Scroll (I, II, III) means its order number, while the star defines its level.
Level 1 (One star)
Level 2 (One golden star)
Level 3 (Two golden stars)
Level 4 (Three golden stars)
How it works
As soon as you study a Scroll for a certain level, the value in the Scrolls counter changes to "1/N". After that, a new element gets highlighted in the Upgrade Menu.
For example, I study "Tunic Scroll ll" with two golden stars - Scroll #2 for Level 3. Then I open the Crafting menu, select Tunic and see that one of the elements on the drawing got highlighted and the counter's value changed from "0/3" to "1/3".
If you find a Scroll that you have studied before, you will receive XP for studying it again.
You can study all the Scrolls at any time you wish. However, you can't skip upgrading levels 2 and 3, even if you have already obtained all the Scrolls for Level 4.