1) Beat the butter and the sugar
The first step is beating together the sugar with the butter. The sugar crystals slice through the butter leaving behind micro pockets of air suspended in the fat. These air pockets help leaven your baked good in multiple ways.
2) Add the eggs one by one
Reduce the speed to medium low and add your eggs one at a time scraping the bowl and any attachments between each addition and mixing until incorporated each time. Adding your eggs one at a time helps keep the emulsion you have going on with the air and fat and sugar.
3) Alternately add the dry and wet ingredients
Keeping the speed on medium low, alternately add in the dry ingredients and the wet ingredients. Because keeping the balance and stability of the batter is important, we always want to add alternately with 3 parts dry and 2 parts wet. Always start and end with the dry ingredients to balance out the liquids of the batter.