The best place for a starter to live is in a glass jar that is large enough (1 gal) and has a non-sealing glass lid. Make sure your jar is clean and well dried.
250 g lukewarm water (to get started, you are likely best to use filtered water)
250 all-purpose flour
Mix well, using a ladle or Dutch whisk and let sit on the counter (at room temperature) for 3-5 days.
It will start to bubble and smell like it is fermenting. (This is good!!)
Remove more than of the starter and ‘feed’ it with equal parts flour and lukewarm water.
(More detailed instructions for feeding and caring for your starter here.)