Thanks.
That’s a rye starter. You can start a rye starter from scratch, but it’ll take some time before it has enough rise for baking. Alternatively you can “convert” a wheat starter by feeding it with increasing fractions of rye flour.
Rye makes for a quite heavy dough, so you’ll want a kitchen machine that’s up to the task.

Nothing special to the process.
I make a pre-dough from 30g active starter, 150g water and 150g rye flour (I use type 1350, but you’re unlikely to find that outside Germany) and let that rise for 12h.
After first rise I add 12g salt, 10g honey, 380g rye flour, 250g water and whatever seeds I feel like.
Let that rise for another 3h, then coax it into a loaf tin (rye dough is awfully sticky, wet hands help) and let it rise for another 2h.
Then bake with steam for 15 minutes at 250°C, then release the steam, reduce temperature to 200°C and bake for another 45 minutes.

It’s based on this recipe but adjusted to my taste and flour. You’ll have to play around and find ratios that work for your flour.