The easiest and most successful method of making your own starter is to combine water, flour, and a tablespoon (or packet) of active dry "domestic" yeast which is available at any grocery store. By letting this brew sit for several days as you would with a dried sourdough starter, the domestic yeast will go "wild" and develop the familiar tang of its truly wild cousins. You'll probably catch some wild yeast in the process as well.
2 cups warm water
1 tablespoon of sugar or honey (optional)
1 tablespoon or packet active dry yeast
2 cups King Arthur Unbleached All-Purpose Flour
Pour the water into a 3- to 4-quart glass or ceramic container or bowl, and add dissolve the sugar or honey and the yeast in that order. Stir in the flour gradually. Cover the jar or bowl with a clean dishcloth and place it somewhere warm. By using a dishcloth instead of plastic wrap, you'll allow any wild yeast in the area to infiltrate and begin to work with the domestic yeast which itself is beginning to develop "wild" characteristics and flavors.

The mixture will begin to bubble and brew almost immediately. Let it work anywhere from 2 to 5 days, stirring it about once a day as it will separate. When the bubbling has subsided and a yeasty, sour aroma has developed, stir your starter once more and refrigerate it until you are ready to use it. The starter should have the consistency of pancake batter.