Wild yeasts exist in the air around you and to some extent on the wheat berries. There are wild yeasts on grapes (unsulphured) and apples and other fruits. It is those wild yeasts which are 'captured' to make a sourdough starter. The process takes from 3 to 5 days. I wish I had specific amounts for you, but you could start with 1/4 to 1/2 cup of flour and mix in enough warm (not hot) water to make a thin paste. DO NOT make it too soupy. That, in fact, is the trick to a good starter, according to the French bread makers, and I think they should know. And after you've fooled around with the flour and water thing, you might wish to branch out into adding those unsulphured grapes, apples, sour milk, etc as a catalyst in order to capture other strains of yeast. Each of these strains has a slightly different taste. In fact if you move to another area, you might end up with a starter that produces an entirely different flavor. For instance, San Francisco sourdough bread is well known and has a distinct taste due to the wild strains in the air there.
On day one you mix the flour and water (and add any catalysts to encourage fermentation) and place in a warm spot. After 3 days, the dough should be moist, inflated, and slightly sour. More flour and water is added (mixed in) and left to sit in a warm spot. After 2 days the process is repeated. Then the next day it is done again. Note the order: 3 days, 2 days, 1 day. At this point you should be able to make a loaf of bread using part of the starter and adding back what you took out in the form of more flour and water.
Rule of thumb: Use about 10% starter to size of loaf. In the case of a 2 lb loaf this is a bit over 3 oz of starter (3.2 to be exact). For a 1 lb loaf 1.5 oz would be used.
A book that describes this process in great detail is The Village Baker by Joe Ortiz, copyright 1993, published by Ten Speed Press, Berkley CA. If it's not still in print, try the used books stores, that's where I got mine. Or try your local library. If they don't have it, they might be able to get it for you.