The cookbook I'm using tells me how to make a sourdough starter, but not how to feed it after I take out what I need for the bread. What do I do with the starter once I've used some? I don't think I'll use all of it at once. 
I feed starter with a 50-50 mix of water and flour.  Usually 1/2 cup water 1/2 cup flour for a small container.  You can eyeball the ratio.  It isn’t critical to get it perfect.    If you keep the starter in the fridge you can get away with feeding it twice a week depending on your yeast strain.  You can do this by eye…if the starter starts to look ugly when you take it out to feed, you should feed it a little more often.   One thing I will say is that you can resurrect a starter that seems long past edible.  The starter will get a cloudy fluid on top of the batter and develop a really strong smell if you let it go too long.  My personal experience is that you can pour off the watery fluid and some of the batter if need be.  Then add a good feeding of flour and water.  It will take a little longer than usual to build up again.  Yeast is a very durable critter.   