I bought whole wheat berries at the health food store and ground my own flour in a coffee grinder from them because I wanted the yeast on the flour to be really fresh, but this probably isn't really necessary. The pre-ground flour at the health food store is probably quite fresh, also, and you can buy very small quantities in bulk.
DAY ONE: Mix 2 Tablespoons whole grain flour and 2 Tablespoons pineapple juice. Stir well, cover and let sit for 24 hours at room temperature.
DAY TWO: Add 2 Tablespoons whole grain flour and 2 Tablespoons pineapple juice. Stir well, cover and let sit another 24 hours at room temperature. You may, or may not start to see small bubbles at this point.
DAY THREE: Add 2 Tablespoons whole grain flour and 2 Tablespoons pineapple juice. Stir well and let sit 24 hours at room temperature.
DAY FOUR: Stir mixture and measure out 1/4 cup--discard the rest. To the 1/4 cup, stir in 1/4 cup unbleached AP flour and 1/4 cup water. Let sit 24 hours at room temperature.
REPEAT Day Four until mixture expands to double its size and smells yeasty. Mixture may start to bubble after a couple of days and then go flat and look totally dead for a couple more days. If this happens, at about Day 6 add the 1/4 teaspoons vinegar with your daily feeding. This will lower the PH and wake up the yeast, which will then start to grow.
Once the yeast starts growing, starter should be fed equal parts of flour and water in a quantity sufficient to make enough starter for your recipe. Store the starter in the refrigerator when you are not using it. It needs to be fed equal parts flour and water once a week to keep it alive. Either use or discard at least half of it when feeding--THIS IS VERY IMPORTANT to maintian a healthy starter! If you forget to feed it for a few weeks, it probably will be fine but may take several feedings to get it back up to par.