Easily avoid some common mistakes to make your cookies turn out perfectly.
Measure out all of your ingredients before you start anything.
Source: antoniotahhan.com
Read the recipe closely, from start to finish. Make sure you have everything and enough time. Baking is finicky; you don't want to start and have to run to the store to pick up some while your already-beaten eggs fall flat. Also: Use real butter.
Don't scoop flour out of the bag with the measuring cup.
Source: marthastewart.com
Spoon flour into the measuring cup until it's piled above the rim, then level it off with the back of a knife or another flat surface. If you dip the measuring cup into the bag of flour, you'll end up with more flour than you should have. Little side note here is that some recipe developers and food magazines prefer the scoop method to the spoon method — there is no industry standard, which is annoying. But if your source isn't explicit, it's usually safer to spoon not scoop. For wet ingredients use a liquid measuring cup.