Add some cultured buttermilk to a non-reactive container such as a 16 oz mason jar.
Add some GF all-purpose flour.
Mix the ingredients well.
Add some filtered water to create a thick pancake-batter-like consistency.
Loosely cover with a beeswax wrap, coffee filter, paper towel, or fabric.
Set in a warm spot.
Every day or every other day stir this mixture.
If brown liquid forms on top of the starter, stir well. If it is very thin, add a bit more of the GF flour and mix well.
When the starter has formed a lot of bubbles, has a pleasant sour smell, and pleasant, fizzy taste, it is ready for baking.