Horchata v0.6b: near-complete success!
1) Make cinnamon infusion. Bring 4c water to a boil in a small pot or pan with a tight-fitting lid. Place 2 heaping teaspoons of ground cinnamon in a disposable paper tea bag and put it in the water, holding it in place with the lid. Boil ten minutes. Discard the tea bag. (DO NOT squeeze it to try to get the last of the liquid out. You'll end up with shreds of wet paper and cinnamon all over the place.) Take the infusion off the heat, let it cool, and store it in the fridge in a container with a tight-fitting lid.
2) Make 1:1 sugar syrup. Put equal parts sugar and water in a pan, bring it to a boil, and whisk until the sugar completely dissolves. Immediately take it off the heat, let it cool, and store it in the fridge in a container with a tight-fitting lid.
3) Make the horchata base. In a blender, coarsely grind 1.5c white rice and then .75c slivered blanched almonds. You want to release the rice starch and almond oils, but you don't want to turn them all the way into powder. Place the ground rice and almonds in a two-quart glass jar. Pour in 4c boiling water and .75tsp vanilla extract. Stir with a long wooden or plastic spoon (carefully--the jar is full of boiling water!) and store in the fridge. Check it twice a day; if the liquid layer on top is still mostly clear, give the jar a good shake and put it back in the fridge. When the liquid layer has turned cloudy white (which will take a few days), it's ready. The longer you leave it, the thicker it will be, so patience is definitely rewarded here. When you can't bear to wait any longer, strain the liquid into a bowl through a regular strainer. Rinse out the jar and strain the liquid back into the jar through a fine strainer. You should end up with about 3c of horchata base.
4) Make horchata! To the base, add .75c cinnamon infusion, .25c sugar syrup, and 2c cold water. If you want it more cinnamony or more sweet, adjust the proportions. To avoid over-diluting, a good rule of thumb is that the total volume of infusion, sugar syrup, and water should be about equal to the total volume of horchata base. Remember that horchata should have a milky texture, and err on the side of less dilution as it's served over ice and the melting ice cubes will dilute it further. Store it in the fridge.
I say "near-complete success" because a) I used 4.5c of boiling water, which I think is too much; b) I strained it after two days and the liquid wasn't quite thick enough yet; and c) I over-diluted it while trying to figure out the cinnamon infusion proportions. But I think the recipe as given above should work basically perfectly.