The Kir Royale was popularized by and named after Felix Kir who was the mayor of Dijon, France from 1945 until his death in 1968. Kir had a fondness for a local drink called a blanc-cassis which was still white Burdundy wine and Creme de Cassis. He served this drink constantly at official city functions - so much so that his name was adopted for the drink, a Kir.
Of course, the French being who they are, a variation came about using Champagne instead of still white wine and this is called a Kir Royale.
Use a good sparkling wine or non-vintage Champagne for this drink. It is mostly wine and though the Creme de Cassis pulls more than it's weight in the drink's flavor profile, a blasé wine will cripple a Kir Royale.
Now, to the Creme de Cassis. You want a French one which used to be very difficult to find. Things are better in this department lately - if I can find two different brands in Kansas you should find be able to find at least one. A half ounce is all you need as Creme de Cassis has a quite concentrated flavor.
Finally, a long, thin lemon twist finishes off a Kir Royale and provides a nice citrus oil aroma and a graceful look as it corkscrews lazily on the flute.
If you have a Champagne/Creme de Cassis combination that works well let us know in the comments.