Add the Creme de Cassis to a champagne flute and top up with sparkling wine. Make sure to add the Creme de Cassis first, or else it will settle to the bottom and not mix as well.
The Kir Royale takes something that's already great, Champagne, and makes it better. Now, don't get me wrong, I'm not going to forego all Champagne just to drink more Kir Royales, but it's definitely in my brunch cocktail rotation.
I'd suggest not reaching for your finest vintage grower champagne for this drink. Instead, use something cheaper, like a Spanish Cava or a French Crémant. There's no use breaking the bank, and this is a good way to liven up a less interesting bottle of sparkling wine.
I tend to like using a little more Creme de Cassis in my Kir Royale than a lot of classic recipes. I have found that when I use a sparkling wine that is more tart or acidic that I prefer using a slightly larger amount, and on a sweeter or less tart sparkling wine you can get away with less. I suggest starting with less and adding more if desired.