These instructions show how to feed a 20% dark rye sourdough starter mix. This is not rocket science, and approximate amounts are fine, though the ratio of water to flour should be as accurate as you can make it. The 20% number refers to the proportion of the flour which should be dark rye vs. white wheat bread flour. The instructions are the same for a 100% wheat starter, just add 50% water and 50% bread flour by weight.
The 20% rye starter will survive for a month or two in the refrigerator without being fed, possibly longer. I typically feed mine every time I make bread, which is about once a week.
 Generally speaking, you want to no more than double the amount of starter by feeding. So if you have 200g of starter, don't add more than 200g of flour-water mixture at a time. I usually add less, aiming for about 120g after making a loaf that takes 100g, figuring I lose about 20g to waste.