Potions class

Is it the first year? If so, then you may be stirring wrong.
When you look at the directions, it tells you what ingredient, what tool, which way to stir(clockwise or counter-clockwise) and the color it should be.

EDIT// Just realized the stirring goes for any year. Sorry about that. ^^'

You need to hammer the first ingredient by tapping on the screen add it to the cauldron then stir in the direction of the arrows on the recipe (left side). Stir till the color starts to change to the desired then stop and wait for a check to appear next to the first ingredient. Green is the first ingredients end color. follow the recipe for the other ingredients paying attention of the direction of your stirring and you should be all set.