How do you become a skull level on WorldofWarcraft

You can't become "skull level".
There are two types of "skull levels". When a player's level appears to you as a skull, it means he is more than 10 levels higher than you. If he is 11 levels higher, and you gain 1 level, he will no longer show a skull - you'll see his actual level.
Raid boss levels are depicted as a skull as well. This simply means that they act as if they are 3 levels higher than the maximum level that a player can attain. There's nothing you can do about that. They are designed to require good gear (and 10/25 players) to defeat.