Can somebody help me with the meta skill mechanic?

I'm having poblems with the meta skill mechanic.
Here is what i have in the skills yml:

Fire:
Skills:
- effect:explosion @Self ~onTimer:4

And here is the item itself:

Flame-spell:
Id: NETHER_STAR
Display: '&4Flame spell'
Lore:
- '&4Standaard fire magic'
Skills:
- skill{s=Fire} ~onUse

And yes i am using Artifacts for this.
I really hope you all can help me with this, thanks in advance!