Such a simple drink. You need crème de cassis (I like Gabriel Boudier), and dry white wine. I keep the cassis in the fridge once it's opened. The syrupy sugar content might preserve it, out, but it's about 20% alcohol by volume, so could spoil. It's not the kind of thing one polishes off overnight. Mine lasts months.
A slosh of the cassis into a glass. The strength is up to you. Top with cold white wine. I add slices of lemon or lime, or, in this case, yuzu - a very perfumed citrus.