Cooks Assistant Guide

Teleport home to Lumbidge and talk to the cook in the castle who will tell you to gather supplies but doesn't know where to get them.

Go to the cow pens east of Lumbridge and milk the Prized Dairy Cow for top quality milk.

Then go to the house north of Lumbridge Castle to the east of the mill and there will be a large egg spawned just outside of the house, inside the fence.

Then go to the mill north of Lumbridge Castle and talk to Millie to make sure you get fine flower. Then grab some wheat and climb to the top of the mill, put the wheat in the hopper, operate the levers, and go back down and collect your flower.

Finally head back to the cook and talk to him to complete the quest.