Start with creating your lemon twist. See instructions above or watch this video tutorial if you're not sure how to make a lemon twist garnish.
Once you're make your lemon twist, take the leftover flesh of the lemon and use that to rub onto the rim of your martini glass in order to make it sticky.
Pour a good amount of sugar onto a flat plate then dip the lemon-rubbed rim of the glass into the sugar until the rim is nice and sugary. (I typically pour the remaining sugar back into my sugar canister when done)
Next, add vodka, limoncello, and cranberry juice to a cocktail shaker with ice and shake. If you prefer a sweeter cosmo, add a splash of grenadine too. Strain into your sugar-rimmed glass and garnish by floating a fresh (or frozen) cranberries in the drink and then rest the lemon twist on the edge of your glass.