For the garnish, moisten the edge of the glass with lemon / lime juice or pastis and then press in sugar.
Shake pastis and lime juice well in a shaker, pour into a glass and fill up with well-chilled mineral or soda water.
Adjust the quantities to the glass size and personal taste - more or less alcoholic. You can replace the lime juice with lemon juice. Be careful with the quantities so that the acid does not become too dominant!