Basically you ‘make’ the yeast yourself, by mixing flour and warm water. It needs keeping warm and ‘feeding’ daily by replacing 50% of it’s volume with more flour and warm water. After about a week you have what is your ‘starter’, living and breathing with yeast which can then be kept in the fridge!
Each time you bake a loaf you mix in more flour and water an leave it out overnight to proof. You then use some of this to bake the loaf, and keep some of it as the starter for next time. So it’s not actually 2:3 yeast to flour, it’s 2:3 yeasty mixture to flour. 🙂
Some bakers have starters which were made hundreds of years ago, as long as you keep feeding it it will live indefinitely!
This bloke explains it better than I can really linky
And yes, mine were made with a tin!