Here's the best way to add flour and other dry ingredients during mixing (if using a hand-held mixer, you'll need to stop it while doing so):

I like to put my flour and other dry ingredients in the middle of a long sheet of waxed or parchment paper, about 12 - inches in length.  

1. First fold the paper lengthwise and open flat. Place ingredients in the middle of the long sheet.

2. With your left (right) hand, support the paper under the left (right) hand edge. With your right (left) hand, hold the top two edges of the waxed paper that come together from folding. 

3. Lift the paper to the edge of the mixing bowl, and shake your hand while titling the paper towards the mixing bowl so its contents empty in a steady stream. Take care not to tilt the paper too much, because the dry ingredients will empty quickly into it.

NOTE: Another way is to add flour and dry ingredients to a mixing bowl is to use a large spoon and scoop the flour and dry ingredients into it. Then, holding the spoon at the edge of the mixing bowl, shake its contents a couple of times, so they empty into it.