I generally use a soup spoon to spoon my batter into the liners to try to avoid spillage, but an ice cream scoop is a popular method as well because it's generally the right size for the cupcake liners! Cupcake batter will rise while baking, so most recipes fill the liners about halfway to two-thirds of the way up to avoid the batter overflowing in the oven. Many recipes will either tell you how much to fill the liners, or when you divide the batter evenly among the number of cupcakes the recipe makes, each liner will only be filled part way.