Using an existing Amish Friendship Bread starter that's been properly fed and maintained, stir well and then remove ½ cup and place in a new jar, mixing bowl or Ziploc freezer bag. If you would like to use up the remaining starter, bake with it, share it, or freeze for future use.
To the ½ cup Amish Friendship Bread starter, add ½ cup flour ½ cup sugar, ½ cup milk, and stir well.
Cover loosely with a dish towel or plastic wrap. If your jar has a latching lid, leave the lid cracked open. Store your starter in a warm, draft-free area of your kitchen.
Stir your starter daily and move to a weekly schedule of baking once a week. On your baking day, feed your starter 1 cup flour, 1 cup sugar, 1 cup milk. Stir well and remove one cup for baking. You should have a little over 1 cup of starter remaining in your jar, bowl or Ziploc freezer bag. If you have excess starter, bake with it, share it, or freeze for future use.
Repeat step 4. If you ever need to take a break, freeze your starter.
Notes
As with all living things, you'll want to take good care of your starter, especially if you're reducing the overall volume on a regular basis. Stir or mix daily (or mash the bag, if you're keeping your starter in a Ziploc), and store your starter in a warm, draft-free area of your kitchen or house. Maintain the proper ratios and if you end up with too much starter, bake with the excess or share it with others.