If you are sugaring the rim of your glass, place the sanding sugar in a shallow dish. You can dip your finger in water and rub it around the rim or you can use a slice of lemon to go around the rim -- you just want it to be slightly damp so the sugar will stick.
Fill a highball glass with ice and then pour the RumChata over the ice. Slowly pour the blue cream soda in to fill the glass.
I like to float just another half-ounce or so of horchata over the top.