Sprinkle yeast over first amount of warm water in medium bowl. Let stand 10 minutes. Stir to dissolve yeast.
Add second amount of warm water, flour and sugar. Beat until smooth. Cover with cheesecloth (I just use a paper towel). Let stand at room temperature for several days (I let mine stand 10 days), stirring two or three times each day. When ready, starter should have a sour smell with small bubbles rising to the surface. The process will take from 5 to 10 days. Turn into jar, cover and refrigerate. I put the cover on the jar, but don’t screw the lid on. Stir your starter with a wooden spoon (wooden only!) every couple of days to incorporate the liquid that will rise to the top. I do it whenever I think about it - not religiously. :)
To replenish starter: for each 1 cup of starter you use, add 3/4 cup all purpose flour, 3/4 cup water and 1 tsp. sugar. Stir well. Cover loosely. Allow starter to stand at room temperature until bubbly. This should be at least one day. Refrigerate.
To feed starter: If starter isn’t used on a regular basis, stir in 1 tsp. sugar every 10 days.
I let my starter sit out of the refrigerator for up to 24 hours after I feed it so it gets nice and bubbly again.