Combine the tequila, grapefruit juice, lime juice, Simple Syrup and orange liqueur in a pint glass. Fill the glass with ice, cover with a shaker tin and shake vigorously for 8 to 10 seconds. Strain into a rocks glass filled with ice. Garnish with cactus fruit peel.
Put the sugar and water in a small saucepan. Cook over low heat, stirring, until the sugar dissolves. (Alternatively, heat in the microwave.) Store in a sealed container in the refrigerator.