Easy!

2 cups all purpose or bread flour
2 cups skim milk (fat can retard growth of yeast)
1 tbs sugar
1 package yeast

Mix all, cover tightly, and let bubble for 24 hours at room temp. Add 1 cup flour and 1 cup milk, let bubble again, do this 3 days. Retain two or three cups, and keep in fridge. Refresh every three days with 1 cup of each milk and flour and every so often add 1 tsp sugar.

Every now and again I see a mix that calls for salt, but this also retards the growth. Water will do if you are out of skim milk, or as I do, add water and the appropriate amount of dry milk.