To activate a scroll the players handbook states:
The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divine spells. (The type of scroll a character creates is also determined by his or her class.) The user must have the spell on his or her class list. The user must have the requisite ability score. If the user meets all the requirements noted above, and her caster level is at least equal to the spell’s caster level, she can automatically activate the spell without a check. If she meets all three requirements but her own caster level is lower than the scroll spell’s caster level, then she has to make a caster level check (DC=scroll’s caster level + 1) to cast the spell successfully.
Use magic device states:
Use a Scroll If you are casting a spell from a scroll, you have to decipher it first. Normally, to cast a spell from a scroll, you must have the scroll’s spell on your class spell list. Use Magic Device allows you to use a scroll as if you had a particular spell on your class spell list. The DC is equal to 20 + the caster level of the spell you are trying to cast from the scroll. In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability. If you don’t have a sufficient score in that ability, you must emulate the ability score with a separate Use Magic Device check (see above).
If a creature like a skull lord wants to use a scroll and has no class levels does he have to make a use magic device check to 1. Emulate a class ability, 2. Activate the scroll 3. Potentially emulate an ability score?
Or is it just the one use magic device check to use the scroll?