Flour must be aerated before measuring because it often settles in the bag or container making it heavy and compact, resulting in too much flour being measured. Aerating basically means fluffing it up and is not the same as sifting. Flour should not be sifted before measuring unless the recipe states to do so. Otherwise sifting will result in too little flour being measured.
If you dip into flour without aerating, you will be getting too much flour and your dough will be too dry. To aerate flour you simply stir it around with a spoon before measuring. To measure, be sure to use a flat-topped dry measuring cup. You can see how I aerate flour in my Easy One Bowl Chocolate Cake video: http://www.jennycancook.com/recipes/easy-one-bowl-chocolate-cake/
After aerating, there are two ways to measure the flour: 1) Scoop & Level – Gently scoop the flour up with a spoon and sprinkle it into your measuring cup until it’s mounded above the rim. Do not tap the cup or the container of flour. Finally, level off the excess flour with the back of a knife. 2) Dip & Level – Gently dip your measuring cup into the flour until it’s mounded above the rim and level off the excess flour with the back of a knife. A properly measured cup of flour weighs 4 1/4 ounces.