Love the no knead sourdough recipe, but I don’t see where it tells you to take some of the dough to make a new start.. please advise.

The recipe assumes that you have an already fed, mature starter. You'll use 100g of it in the recipe, and what you have left will be the part that you feed to continue your starter. Look here https://www.kingarthurflour.com/recipes/sourdough-starter-recipe for instructions on feeding!