Requirements:
1 skill
1 common event
For this tutorial, lets use Heal as an example:
1)Create a common event, name it "Heal"
2)Create a "Heal" Skill.
3)Tag the "Heal" Skill into the common event.
Code:
Under the common event "Heal" Put these codes in:
@>Conditional Branch:[Blue Ruby] in inventory
@>Change Items:[Blue Ruby] - 1
@>Text: You used 1 Blue Ruby to execute the skill
@>
: Else
@>Text: You need 1 Blue Ruby to execute the skill properly. You Lost 100hp.
@> Play Se: *Put sound effects if you like*
@>Change HP:[Gloria], - 100
@>
System: This code was tested on rmxp, this should also work with 2k3 or 2k.
Final Notes: You can add more requirements obviously.
There you go, this could also work with my previous tutorial called: "Skills that levels up" 
