First make the simple syrup. Add the water and sugar to a small saucepan over medium heat. Stir until the sugar is dissolved and just begins to boil. Then turn off the heat, add the rose water, and stir to mix. Leave the syrup to cool or transfer to a lidded container and chill until you are ready to use it.
Garden Party Cocktail: Method
Measure out and add everything but the tonic water to a shaker.
Top with ice, close the shaker, and vigorously shake everything for about 30 seconds.
Double strain using a hawthorne strainer and a fine mesh strainer into either a coupe glass (sans ice) or a rocks glass (with a large ice cube). Top with tonic water and garnish with a lime wedge, an edible flower, or dried rose buds to serve.
Notes
Very Important! When purchasing rose water, make sure you buy rose water and not rose extract or essence. Rose extract or essence is roughly 3 times more concentrated than rose water and can sometimes taste medicinal or chemical. IMO they aren't a great substitution for liquid applications and you really need this specific ingredient.