He means you have to create a spellbook ability that contains all the spells
- spellbook Level 1 contains ability 1 level 1, ability 2 level 1 etc...
- Same spellbook ability but level 2 contains ability 1 level 2, ability 2 level 2,etc...
where ability 1 and 2 are the abilities you want the hero to have. But you have to create a copy of each of these abilities for each level you want it to have.
For example, if xou have 2 abilities with 3 levels each, you need to create 6 abilities: ability 1 level 1, ab. 1 lv2, ab 1 lv3, ab 2 lv 1, ab 2 lv 2, ab 2 lv3, and the spellbook ability. Now you add these spells to the spellbook, the level 1 abilities to spellbook lv1, lv 2 abilities to book lv 2, etc.
the downside is you have to create alot of abilities which is boring and takes some time.
Another way is to levelup the spells inside the spellbook via trigger - just use
Unit-set ability level of (ability) for (hero) to (level)
you have to find a way to let the player choose which skill to increase.(dialog or something, or increase all the abilities inside on levelup, or what you want them to levelup.)
This way, you just need the spellbook ability and the abilities inside once, each with multiple levels.