Place the rice, barley and cinnamon sticks in a large saucepan and cover with 4 cups water. Bring to a boil and then lower the heat to a simmer and cover.
Simmer, covered, for 20 minutes. Add more water if needed to prevent sticking to the bottom. Once the rice and barley are cooked turn off the heat and stir though the sugar, vanilla, nutmeg and another 3 cups cold water.
Remove the cinnamon sticks and blend in batches.
Pass the blended liquid through a strainer into a large bowl, using a spoon to press down and squeeze as much liquid out as possible. You'll end up with a bowl of thick liquid and the mushy solids that you couldn't strain any more liquid out of (put these aside in a container and eat them for breakfast as porridge, so delicious!).
Stir the remaining water and the rice milk through the liquid that you strained out of the barley and rice. If you like you can thin it out a bit more with some more water, it's up to you.