Enter the inventory screen and press the Right Mouse Button when over any potion. Select the "Drink Potion" option and quickly pick up the potion with your pointer when you hear the drinking sound, but before the number indicating the number of that particular potion on the inventory screen decreases. If done correctly, you will be able to move the potion to an empty inventory square. Once the potion is placed there, select any scroll and place in the square that the potion was originally located in. Return to the game, and your character will cast the spell from that scroll.