Flaked grains go through a process of being steamed and rolled to gelatinize its starches. This process makes the starches readily soluble and digestible by the naturally occurring enzymes in barley malt. Because the grain is un-malted, it does not possess the enzymes required for this conversion by itself and it must be mashed with malted grains. All grain brewers can add the flakes directly to the mash with other grains. It's advisable to use Rice hulls in the mash when using large percentages of flaked grains in order to help prevent stuck sparges.
Extract brewers must perform a partial mash in order to use flaked grains. A ratio of 1:1 mixture of flaked grains to barley malt is usually sufficient. Use 6 row barley when possible.