I can't seem to get the hang of using a magic scroll successfully. Do I press & hold both mouse buttons, click one, or what?

It's pretty much like casting a spell. You equip it and cast (depending on spell you hold the button down. Blizzard takes a longer hold than, say, firebot.

And you just hold down the button for the hand you have the scroll equipped in. Some spells you only equip in one hand, but uses both hands in the animation to cast.

The animation depends on the type of spell the scroll casts. So if it's a scroll of Fireball, you will equip it (say into your right hand), hold down the left mouse button until it's ready to release, and then release the button while aiming where you want it to go.

Scrolls of Master-level spells such as Fire Storm require both hands to cast, and they also require a longer casting animation. So you'll equip the scroll as above, but you'll need to hold down the button for longer, then release when your hands are done waving.

It took me awhile to figure out what I was doing, too. Just play around with it, preferably not in a really dangerous fight, until you get the hang of it!

Scrolls are two part. You equip it to be able to cast the spell, then you cast the spell like any other magic.