Instructions
Leave the cottage where you acquired the first ingredients needed for the potion. You will see a ghost horse standing to the right of the building.
Scroll to the "Sugar Cubes" in your inventory. Click and drag the cubes to the ghost horse, then click the horse to give it the cubes. This action will make the horse move from the right side of the screen to the wagon resting on the left and automatically harness the horse to the wagon.
Wait for the horse to pull the wagon toward the center of the screen and disappear.
Move your cursor over the ground where the wagon previously rested until it turns into an eyeball and then look at the ground.
Click the first earthworm you see using the skeleton hand cursor to add the earthworm to your inventory. Repeat with the other three earthworms and the salamander nearby.