I usually sift the flour into the batter to reduce the chance of lumps. Stir until the flour is all combined and you can't see any white bits. Then stop mixing - you don't want to overmix and risk getting tough cake. It may still look like there are some lumps, but that's OK. You won't see or taste them when the cake is baked. If you don't have a sieve, just sprinkle the flour over and stir in little by little.