Gather your materials that I listed in A Sourdough Experiment.
3/4 cup (6 oz can) room temperature, canned, unsweetened pineapple juice
A quart sized, wide mouth canning jar (or other similar glass container)
1 cup whole wheat flour
A wooden spoon (never use metal)
something to cover the top**, a mesh stocking, paper towel, cheesecloth, coffee filter etc (think breathable material)
Mix the pineapple juice (room temperature!), and flour in a small bowl until combined.
Place your jar in a safe spot on the counter for 24 hours. It should be in a room temperature area, about 76-80 degrees. (Although room temperature is not a scientifically defined term) Just don't have the AC on blowing right at your precious jar!
**I decided to change the cover on the jar on day 2 to a two pieces of tulle because I wanted something more breathable than a paper towel.