Here’s a useful yeast starter that won’t turn sour. To initiate the starter, add 1 teaspoon of normal baker’s yeast to the following feeding mixture and incubate in a warm place:
3 tablespoon instant mashed potato flakes
1/2 cup sugar
1 cup lukewarm water
Every week (or even every few weeks will work), take your starter from its habitual resting place in the refrigerator, let come to room temperature, and add the above mixture (no further yeast needed). Let stand several hours (or overnight). It should get very bubbly.