Very simple and easy achievement.
You will require 3 buckets of milk, 3 wheat, 2 sugar and an egg.
Buckets of milk come from cows, use a bucket on an adult cow and there is your milk.
You can grow wheat using seeds, or find a village with it growing.
Sugar is from sugarcane, 1 Cane = 1 Sugar. Sugarcane can be found along the edge of water.
Lastly, eggs come from Chickens. Which seems a bit obvious but ive written it anyhow.
Simply have the required items in your inventory, go to a crafting table and choose the Cake! Manual crafting is this -
Milk Milk Milk
Sugar Egg Sugar
Wheat Wheat Wheat
Each new word being a space in the crafting menu.