As I have stopped eating sugar and most commericial soy, oat or rice milks are made with added sugar I have attempted to make my own "milk". For my first attempt I started with brown rice.
Take 1 cup rice and boil it for 1 hour in 2 cups of water and 1/4 teaspoon each of cinnamon and cardamon. Watch towards the end as the water cooks out completely and you don't want it to burn.
Once cooked add 2 cups fresh water and blend the whole thing with a stick blender. Pour into a colander lined with muslin and place over a bowl. Keep it in the fridge for 1 hour. Once cooled you can strain it by "milking" it with your hands to remove the liquid from the pulp. At this point it will look too thick so add another 2 cups of water. If you are happy with the consistency you can stop at this point. For a runnier milk keep adding water until you reach something you will be happy with. I ended up with 2 litres in total. It has a lovely taste that reminds me of rice pudding.