There are many ways to make gravy for mashed potatoes. To make a serving of one cup of traditional brown gravy you will need: 1 cup of beef bouillon, 2 tablespoons of flour, 2 tablespoons of butter, and salt and pepper to taste.
To begin, melt the butter in a saucepan over medium-low heat, and stir in the flour. Continue stirring and cooking until the mixture is well blended and golden brown, and add the beef bouillon.
Stir until the gravy reaches boiling point and the consistency is smooth and the gravy has thickened. Finally, add a dash of salt and pepper, taste and serve.
Gravy may be made with melted butter and flour as above (this combination is known as a roux and acts as the thickening agent for many different sauces), or alternatively a slurry can be used.
A slurry is made by taking the drippings from the food being prepared, thickening it with corn starch or flour and adding it back into the stock to boil.