There are a number of recipes for sourdough starter, but this is one I've used, and it works fairly well. It came originally from the cookbook The Complete Sourdough Cookbook by Don Holm.
If the starter starts turning orange or green, throw it away. Other nasty things have started growing in it!
After the starter is ready, it can be used immediately. Try to use some of the starter once every few days, or it will start to die. The starter can be refrigerated to last longer (use at least every other week). Make sure the starter is at room temperature before using.
After using some of the starter, replenish what you take with equal amounts of warm water and flour. Let the starter sit until nice and bubbly before using again.
Usually, the first starter made will need a while to "come up to strength." Best bets for first recipes are ones that don't require much raising, such as pancakes, or ones that have additional yeast.
Bill Turner Hewlett-Packard Personal Software Division, Santa Clara CA {hplabs!}hppsdc!billRecipe last modified: 2 Jan 86
From: bill@hppsd2 (Bill Turner) Newsgroups: mod.recipes Subject: RECIPE: Sourdough Starter Date: 16 Jan 86 08:36:14 GMT Approved: reid@glacier.ARPA Organization: Hewlett-Packard Personal Software Div., Santa Clara CA Copyright (C) 1986 USENET Community Trust Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the USENET copyright notice and the title of the newsgroup and its date appear, and notice is given that copying is by permission of the USENET Community Trust or the original contributor.