Adjust an oven rack to the middle position and place a loaf pan or cake pan in the bottom of the oven. 2. Place the container of dough on the middle rack and pour 3 cups of boiling water into the pan. 3. Close the oven door and allow the dough to rise as instructed. If you limit the time that the oven door is open, the proof box can be used for both the first and second rise without the need to refresh the water.
From Cook’s Illustrated – Turning Your Oven Into A Proof Box