If you're making nut milk, cover the nuts with water and soak overnight. Skip this step if using oats or rice.
Drain the nuts if using them. Add them (or your oats or rice) to a blender with 4 cups of filtered water, maple syrup if you're using it, and any flavorings. For vanilla milk, 1 teaspoon is a good place to start. For berry milk, add 1/3 cup of berries. For chocolate milk, add 3 tablespoons cocoa powder. You can adjust the flavors and sweetness to taste once it's all blended.
Process on high for 1 minute. Taste and adjust as needed. If you prefer a thinner consistency, you can add more water at this point, too.
Pour the milk through your straining medium.
Transfer your non-dairy milk to an airtight container and refrigerate.
I highly recommend using a nut milk bag! You can set it in a bowl and pour the milk through it, which is much easier than dealing with a piece of cheese cloth if you're making non-dairy milk on your own. Plus, if you're saving the pulp for smoothies or baking, it's easy to scoop out of the bag.
Non-dairy milk will keep for 3-4 days. It's normal for it to separate in the refrigerator—just give it a shake and it's ready to drink!