Let's se if I can remember here...
Step 1: Get any number of the same type of scroll (so they stack)
Step 2: Get an item you want to clone (I'm pretty sure there's some you can't though)
Step 3: Swing your weapon, then, in mid swing, open your inventory
Step 4: Go down to the stack of scrolls and double click them
Step 5: Drop the item you want to dupe, this should create a number of items equal to the number of scrolls you used
Step 6: Pick the items up
Note: This is for oblivion, just incase you were unsure
It is hugely unbalanced, as soon as you leave the tutorial, chances are you'll have 2 of one type of scroll and one of another, using them you can create more and more scrolls, then dupe hundreds of items at a time. I will admit though, it can be useful at times.