Good luck! It is hard to maintain and very finicky. If you're up for the challenge, here it is.
Heat the millk and add water and cool to luke warm. Stir in sugar, salt, flour, and beat. Pour into a large bowl (allow for expansion), cover with cheese cloth and allow to stand 3-5 days in a warm spot, until mixture is bubbly and has a sour aroma. Dissolve yeast in 1/2 c water and mix into starter. Cover and stir down each day for a week.
Several hours before, prepare the sponge and allow to rise to double.
Dissolve dry yeast in 1/2 c water and add to sponge. Add 1 c flour mix and turn onto board. Knead in additional flower to make a stiff dough.
Place in bowl and rise 1-1/2 to 2 hrs. Punch down and rise 45 minutes. Divide dough in half and shape as french bread and allow to rise.
Bake at 400° 30-40 min.