On low speed, add in a third of the flour mixture and mix, then add half of the buttermilk mixture and mix some more. Next, add half of the remaining flour, mix, the rest of your milk, mix, and finally the rest of your flour.
You want to start and end by mixing in flour because this insures the ingredients don’t slosh around and that everything is incorporated well. Never add all of the flour or milk at once, since this can lead to overmixing and a very dense batter!