This article is from the Sourdough Starters FAQ, by Brian Dixon briandixon at hotmail.com.
1. Using 1 tablespoon of starter (discard unused portion or save a
little in the refrigerator in case of an emergency), 1 cup 75 degrees
water, and 1 1/2 cups all-purpose white flour, proof for exactly 24
hours at 72 to 77 degrees. It's very important to maintain these
precise temperatures and to proof for exactly 24 hours.
2. Examine the starter to determine what stage it's in. Assuming you
didn't overheat it, it should be "flat", "barely living", or
"healthy". Remember the clues to identifying non-healthy starter (
low number of bubbles, early hooch, gelatinous consistency, no froth
on top, or any 2 or more of these symptoms. If your starter is
"healthy," you're done.
3. If your starter is not healthy yet, stir it well and refrigerate
it for no less than 12 hours.
4. Remove the starter from the refrigerator and go back to step 1.
This process needs to be repeated a few times ( usually around 4 or 5
times or so unless you were lucky. A lot of the home-dried starters
revive MUCH quicker than this.
Here's an alternative process you can use (possibly better, if the
above process doesn't seem to work well for you):
 
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