Soak the rice and cinnamon stick(s) in 4 cups of water over night (at least 8 hours). *I use 2 cinnamon sticks when making this for my mom, who LOVES cinnamon. You can use just one if you don't want a strong cinnamon flavor.
After soaking, pour everything into a high-powered blender. If it is large enough, add 4 more cups of water. If your blender isn't large enough, you can add the additional water after straining.
Blend on high for 2-3 minutes, or until the rice has pulverized into a fine powder.
Strain the horchata using a fine mesh strainer and a triple-layered cheese cloth. Discard the rice and cinnamon. Add the additional 4 cups of water at this time, if you didn't already add it to your blender.
If you don't like a super grainy texture (some people do), I recommend straining at least twice.
Add 1/4 tsp of vanilla extract. I do not add vanilla extract to my horchata made with dairy, but I find that it adds a nice boost in flavor in this water-based horchata.
Add the sugar and mix thoroughly.*Pro tip: when adding granulated sugar to beverages, it's much better to turn the sugar into a simple syrup first. It really makes a difference in flavor in my opinion- all you need to do is heat 1 cup of sugar with 1/2 cup of water in a saucepan over medium low heat for a couple of minutes.
Pour over lots of crushed ice and enjoy :)
Notes
This will last in the fridge for up to 5 days. Give it a good mix before pouring into your glass.