Traditionally, the Boulevardier is 1 ounce (1 part) each Campari, sweet red vermouth, and rye whiskey or bourbon, but many versions use 1.25 to 1.5 ounces (1.25 to 1.5 parts) of the latter, and I go even further to 2. (Bourbon is generally more mellow and sweet; rye, slightly more spicy and dry.) It is not a subtle drink. It should definitely be sipped quite slowly. We make it on the rocks, but you can also shake it with ice, strain it, and serve it up. I show it here with an orange peel we like very much (a cherry, and yes, the Luxardo ones are absolutely worth it, is also good) but it’s also good with no garnishes whatsoever.
In an old-fashioned glass, mix everything and add ice to taste.