Add ice to your glass (I like to use a short glass).
Add the St. Germain then top with club soda.
Stir gently and garnish with the orange slices.
Notes
Variations:
Cucumber - cucumber pairs really well with elderflower so a few slices or ribbons made with a vegetable peeler added to the glass would take this cocktail to the next level.
Herbs - fresh herbs like basil, thyme and rosemary would add a beautiful aroma garnishing this elderflower soda. Be sure to bash the sprigs a little between your palms before adding them to your drink to help release their aromas.
Citrus - slices of lemon or lime would work well here instead of the orange. To add a little more acidity to your drink squeeze in a little of the citrus juice before stirring the drink.
Flavored Water - There are tons of tasty flavored sparkling water options on the market. Try one that would pair well with the floral taste of the liqueur.