Sugar the rim of your martini glass by cutting a slit in your lemon wedge and running it around the rim. Then with the sugar on a plate. wiggle the glass on the sugar to form a nice crunchy rim of sugar.
Fill a martini shaker with the ice
Add in the vodka, simple syrup, lemon and grapefruit juices
Let sit for about a minute to chill
Shake vigorously then pour into your sugar rimmed martini glass