Allow the yogurt bacteria to incubate. Keep the yogurt warm and still to encourage bacterial growth, while keeping the temperature as close to 100ºF (38ºC) as possible. The longer the mixture incubates, the thicker and more tangy the yogurt will be.
Keep the yogurt still during incubation. Jiggling won't ruin it, but it makes it take a lot longer to incubate.
After seven hours, you'll have a custard-like texture, a cheesy odor, and possibly some greenish liquid on top. This is exactly what you want. The longer you let it sit beyond seven hours, the thicker and tangier it will become.