2ozsparkling water/club soda60ml (or a little more)
1tspsugar optional, approx - or a little more to taste
If you want to have a salt rim on your glass, rub the edge of the glass with either a slice of grapefruit or lime and dip the top of the glass in salt.
Put a good few ice cubes in a glass (traditionally you use a high ball but I have used shorter, which holds around half this quantity). Add the tequila to the glass, the lime and grapefruit juice (with sugar mixed in if you prefer sweeter). Top up with sparkling water/club soda and serve.
As mentioned above, this is often made with grapefruit soda rather than grapefruit juice + soda water, so if you have that you can use it instead.