Reactivating dehydrated sourdough starter is simple.
Day one. Rehydrate.
Place 10g of starter in your jar. Add 20g of water. Stir and rest about an hour to soften the flakes. Then add 20g of flour and stir until no dry pockets remain. Cover loosely and set it somewhere warm. 72 to 78 degrees is the sweet spot.
Day two. First refreshment.
Look for a few small bubbles. Discard 40g, keeping 10g. Feed it 20g of water and 20g of flour. Stir well and mark the level on the jar. Those early bubbles mean fermentation has begun.
Day three. Build strength.
Now you want bubbles throughout and a fruity, tangy, yeasty aroma. Repeat the day two feeding. If it doubles in 4 to 5 hours, it's getting hungry. Feed it more often. Watch the rise line to see where it peaked.
Day four and beyond.
Is it ready? You're looking for four things. It consistently doubles after feeding, shows bubbles throughout, has a predictable rise and peak, and smells clean and fermented. Most active starters double in 4 to 8 hours in a warm kitchen.
When it doubles like clockwork, it's ready to bake.