Speak to the cook in Lumbridge Castle. He is located on the first floor. He tells you about his cake problem. Say that you will help him and he will ask you to get him some milk, an egg and flour.
Milk: To get the milk, use an empty bucket with a Dairy Cow. You will find Dairy Cows in the cow pen north of Lumbridge.
Eggs: To get the eggs, walk north of the cow pen until you come to a chicken farm. In the northwest corner there is an egg respawn. Wait for one to show up.
Flour: For the flour, pick a piece of wheat in a field North-West of Lumbridge. Then, go to the top of the Windmill which is just East of the field. Use the grain with the hopper and operate the controls. Then climb back down to the bottom and empty the flour bin by clicking on it.
Now, go back to the cook, talk to him and give him the ingredients. Congrats!
Rewards- 300 Cooking XP
You can now use the stove in the castle -
1 Quest Point