To make the sourdough starter:
In a bowl mix 1 cup of gluten free flour and 1 cup of active kombucha.
I used a mixture of gf flours, combining rice flour, chickpea flour, tapioca flour and gf bread mix. When it got quite wet and runny I added pysilium husk. Nut and seed based flours don't seem to work, so best to avoid and use those for the actual bread.
Mix well.
Cover the bowl and allow to sit somewhere warm, but not too hot.
Check your starter daily. You will need to feed it (like you would your pet cat). Each day add 1/2 cup each of flour and kombucha or water, mixing it well.
Your starter will be ready when it rises within 3 hours feeding, has wee bubbles appearing on the surface and should smell yeasty.
If you make your own kombucha, you can use the yeasty scum on the bottom of the booch jar instead of kombucha.