In a pot, heat the water and sugar and whisk until the sugar is completely dissolved and the mixture is clear (if it's still kinda cloudy, it's not completely dissolved yet).

To make the sour mix, use 1 part simple syrup and 1 part lemon/lime juice. E.g. to make 1 oz sour mix, mix 1/2 oz simple syrup, and 1/2 oz lemon/lime juice. =)

The simple syrup can keep pretty much indefinitely on the shelf, but if you're a little paranoid like me, add about 1 oz of vodka to the bottle to make sure it doesn't go bad.

I suggest making the sour mix "to order", because fresh citrus juice is always better for the cocktail.