Adapted from Ruth Allman's original recipe published in Newsminer
In a medium-sized saucepan add potato chunks, water to cover, bring to boil over high heat, then lower heat to maintain a slow simmer. Cook until the potatoes are so tender they fall apart, about 30 - 45 minutes. Cool and purée with an immersion blender. You'll want 2 cups of thick "potato water", so add additional water if too much has evaporated during cooking.
Add to the 2 cups "potato water": the sugar and flour, and beat until you have a smooth, creamy batter. Place batter in a ceramic bowl and cover with a clean kitchen towel or piece of plastic wrap. Allow to ferment at warm room temperature for at least 3 days (I keep mine on top of a radiator), or until you see lots of bubbles. (Refer to the original recipe linked in post for more details.)
At day 3, you're probably ready to bake! If you don't see lots of bubbles, feed the starter and wait 12 - 24 hours. Your starter is "active" when it smells acidic and you can see lots of bubbles.