Sift matcha powder into a small bowl and add hot water. Use a bamboo whisk to whisk back and forth in an M shaped motion, until nice and frothy (about 30 sec). There should be no clumps remaining.
Fill a 12 oz glass with ice and watermelon juice, leaving enough room in the glass to add the shot of prepared matcha. Pour the matcha shot in.
Using a microplane, grate 1-2 slices of frozen watermelon on top of the drink to finish, just before serving. Stir and enjoy!