Weigh 30g of dried starter flakes and add to a glass measuring cup or small bowl. Add 60g lukewarm water, and try to submerge all flakes.
Over the next two to three hours, stir the flakes every half hour or so. Eventually they will completely dissolve.
Once the flakes have disappeared, stir in 30g all-purpose flour. Cover lightly with a kitchen towel and let it sit somewhere warm, like your oven with the light on, or a warm sunny room. Leave it alone for about a day.
The next day, take a look at your starter - you're looking for small bubbles. If you see some, or lots, great! Move on to the next step. If you see none, return it to its warm place for a few hours, and check again.
Feed your starter with 30g flour and 30g water. Don't discard any; it's not ready for that yet. We're still trying to build it up. Mix well, and put your starter back in its warm place. Watch for more bubble activity - this could take anywhere from six to 12 hours depending on, well, many factors. If you're seeing lots of bubbles, move on. If you're seeing none or only a few, wait a couple hours and check again.
Repeat the step above. Feed with 30g flour and 30g water, don't discard, mix well, and put it back. Keep an eye out for activity. This time, you want to see lots and lots of bubbles. This could take anywhere from six to 12 hours.
Discard all but 120g of your starter. Feed it using 120g water, 60g all-purpose flour and 60g whole-wheat flour. Stir to mix well. Placed in the oven with the light on, it will double or triple in size in about three to four hours. Your starter is now alive and healthy - time to get baking!
This is a sourdough boule I baked with my revived starter!