Combine ¾ cup flour and ½ cup of warm water in a glass or plastic container. Make sure the container can hold about 1 litre, to avoid overflow.
Stir it vigorously to incorporate air; cover with a breathable lid.
Let it sit in a warm place at around 73 °F/23 °C, for 12-24 hours.
Feeding every 12 hours will increase the speed at which the organisms will be multiplying; if you feed every 24 hours it will take a bit longer but it might be easier to sustain.
After the first 12 - 24 hours you should start seeing some bubbles, indicating that organisms are present.
Repeat the feeding with ½ cup warm water and ¾ cup flour.
Stir vigorously, cover, and wait again for another 12-24 hours.
Before feeding the starter every 12-24 hours, remove half of the starter and discard it. Feed with ½ cup warm water and ¾ cup flour.
After about 5-7 days the sourdough starter should have enough yeasts and bacteria to be used for baking.