Pour a bit of your milk into a mason jar (about 1/4 cup). Add all of the cinnamon & mix with a fork well (until there aren't any clumps of cinnamon left).
Add the remaining ingredients into the jar. Shake until mix is well-incorporated (this is especially important for the cinnamon).
Shake vigorously before use.
If you're having issues getting the cinnamon well-incorporated into the mix, let it sit overnight before use. The horchata mix will meld better that way! You can also try blending it before use.