Cook's Assistant

Flour: Get a pot in the castle, near the location of the cook, or if you prefer you can buy one from the General shop in Lumbridge. Follow the path northwest, past the various farms, and you should find a windmill. Pick some grain from the field nearby and use it with the hopper on the top floor. Operate the hopper controls and go to the bottom floor, there you will find the flour in the bin. Click 'Empty bin' or use your pot on the bin to obtain the flour.

Egg: You can obtain an egg at the chicken farm on the route to Varrock, to get there, first cross the bridge over the river, and follow the road north past the cow patch. You will find a chicken farm on the west side of the road, in the chicken coup there should be an egg, pick it up.

Milk: To obtain the milk, first you need a bucket, the best place is from the basement of the castle, go down the ladder near the Cook and you can find the bucket on a table near the sink. Or if you'd prefer then you could always buy one from the General shop. To get the milk itself, go to the cow patch near the chicken farm you got the egg from, use the empty bucket with the dairy cow there, or click on the 'Milk' option.