You follow the recipe. Following is my favorite recipe for Oatmeal raisin cookies.
Preheat oven to 350 In a medium mixing bowl:
Cream (mix until soft) 1/2c. butter
Add and cream well: 1/2c sugar, 1/2c. brown sugar (if you don't have brown sugar, then add 1c. of sugar and some molasses)
In a separate bowl beat together: 1 egg (if you're making a half recipe, beat the egg, then take away half, which you can cook up), 1 tablespoon (tbs.) milk, 1 teaspoon (tsp.)vanilla extract. Add this to the bowl and mix until smooth.
In a third bowl, sift together (if you don't have a sifter, mix with a table fork), then add: 1c. flour, 1/2tsp. baking powder, 1/2tsp. baking soda, 1/2tsp. salt.
After this is mixed in, add: 1c. uncooked oats, 1/2c. raisins (you could add 3/4c. chocolate chips instead).
Place cookies on a greased cookie sheet about 1 inch apart. Bake for 10-12 minutes or until golden brown.