Start by juicing your limes and oranges. I used roughly 2 1/2 limes and 2 naval oranges per drink. It's a lot of juicing but actually kind of fun to make completely natural margaritas. I just used my citrus squeezer for the whole juicing process and it worked great!
Add all the ingredients (except the sparkling water) to a jar or a shaker, and shake.
Rub a lime wedge around the rim of two glasses. Pour a bunch of salt on a plate and lightly press the rim of the glasses into the salt.
Pour the mixture evenly among the two glasses, then top with the sparkling water.
The mixture will store nicely in the fridge but I would suggest not adding the sparkling water until just before you pour the margaritas. Make sure you shake the margarita mix well before dividing it up if it has been stored in the fridge for a little while.