How to Feed a Sour Culture
The feeding/maintenance of your starter is the same regardless of flour.  Remove and discard (or use elsewhere) half or more of starter.  Add fresh flour and water in the desired proportions.  Stir.  As your starter matures, it becomes acidic.  Too much acidity interferes with yeast activity in the bread making process.  That is why you need to remove a portion of the "spent" starter each time you feed.