Most croissants use a commercial yeast, but for this sourdough croissant recipe all you need is the active starter. Making your starter is very simple but does take some time. The whole procedure will take about five full days.  Day 1: You will need equal weights of flour and water. A good place to start is with 113 g flour and 113 g water. Stir well with a wooden spoon so that no excess flour is left unmixed. Cover loosely and let sit at room temperature overnight.  Day 2: The next day you may start to see a bit of growth in your starter. Discard 113 g starter and feed the rest with a fresh 113 g flour and 113 g water. Use cold water if your home is warm. Cover and rest again for 24 hours. Day 3-5: By day 3 you will keep 113 g sourdough starter and discard the rest. Again add 113 g water and flour. Mix, cover, and rest for 12 hours. Repeat every 12 hours for day 4 and 5. You will know your starter is ready when it is risen and very bubbly.