Step 1: What You Need
Step 2: A Waiting Game
Now for the long bit. The bacteria in the buttermilk are mesophilic, meaning they thrive in room temperature conditions. So take the lid off the buttermilk, place a tissue over the lid, to keep dust out, and set it aside overnight. This lets the bacteria reproduce at optimal conditions so that the buttermilk will have the maximum population, key to making cheese.
The yoghurt is different. It's bacteria are thermophillic, they thrive in slightly warmer conditions than that of the mesophilic bacteria. I'm not sure if you leave it out overnight, I bought naturally set yoghurt. This means that the bacteria have reproduced and converted the lactose sugar into lactic acid, decreasing the pH of the milk and thickening it. Just like why we are leaving the buttermilk out.