If you are using the skill book for the first time (that is, you have not enabled skills before)

Then select the skill you want to enable in your backpack and click on the master skill book (note that it is the one with stars at the top of the master skill book).

Left-click, and then select Use. The defense card skill next to your left hand will show that you have turned it on.

If you want to improve your proficiency, turn it off. (For example, you want to open the third line) Every time you put the third line, your proficiency will increase 1.

When the level is LV3, use the Ultimate Skill Book (with two stars at the top).

When the level is LV6, use the Ultimate Skill Book (with three stars at the top).

Have a good game.