To make blueberry lavender syrup, I simmer one part sugar, one part water, a handful of blueberries, and a teaspoon of dried lavender. I strain out the solids after about 15 minutes.
If fresh lavender isn't available, food-safe dried lavender works well.
I prefer a dry gin, but a floral gin can add even more aroma.
For a mocktail, I leave out the gin and add a bit more syrup and soda.
I always taste and adjust the syrup or lemon juice to my liking before serving.