Click on the customers to see what they want, then click on the potion scroll at the top right and put it in the cauldron, gather the necessary ingredients, and light the fire under the cauldron. Give the finished potion to the customer and collect the money to make room for other customers. Click the lever next to the cauldron to cancel your potion if you've made a mistake, and if you've picked up the wrong ingredient, you can feed it to the garbage creature.