Warm butter in a pan, then add milk and salt when butter is melted (about body temp, almost like for dry yeast).
Mix milk and starter mixtures together.
Add flour and stir til roughly combined.
Let sit for 20-30 minutes then knead until smooth. Add some extra flour if needed. It should be a little sticky but not gloppy.
Let rise until doubled. This depends on house temp and how active your starter is, about- 6-12 hours.
Shape 2 loaves. Let rise until doubled again, about 2-3 hours.
Preheat oven to 375°F.
Bake for 40 minutes, and I turn the loaves around halfway. They should be about 180°F with an instant read thermometer on the inside to know they are done baking.
Notes
On your starter: I recommend feeding 50g starter with 100g water and 125g flour 8-12 hours before you want to make this like a levain/preferment method. Use this OR use 250 grams of starter alone at time of mixing.
Milk is best, but if you swap in water I'd add more butter and sugar to make up for it, a tablespoon or so of each to your measuring cup before adding water.