Take an old fashioned glass and wet its edge with lemon. If you are pressing grapefruit for its juice, then use the grapefruit to wet the edge.
Spread some salt on a plate, and plate the wet glass edge on the salt so that it sticks. You may have to rotate the glass to create an even salt crust.
Shake the glass gently to remove any excess salt.
Pour the vodka and juice into a shaker, shake for 10 seconds.
Put some cubes in the old fashioned glass and strain in the cocktail.
Garnish with a grapefruit wedge and maybe a sprig of rosemary.