You will need:
Decent bourbon (or whiskey... yes, you can use Jack Daniel's). I like a high proof bourbon myself (thanks, Chris Neseman), and since I'm a poor writer, I tend to use the Kirkland variety from Costco. Impossible to beat at twenty bucks per liter.
Bitters. Any brand will do, far as I know. I like to use both the traditional aromatic flavor and some orange. Both are common these days.
Agave nectar. I like it better than sugar because it dissolves easily and I like the taste. I also think it may be healthier, if you care about such things.
Maraschino cherries. I buy the cheap kind for Old Fashioneds. I do love the Luxardos, but I would save those for a recipe in which they won't get all smushed... a Manhattan, for example.
An orange. Not essential, but I like a little twist of orange peel as a garnish (aroma and appearance).
A nice cocktail glass. Seek out vintage glasses if possible. they're not hard to find and you deserve it.
Factory-made ice. Yes, it's important. Don't use those disgusting, opaque half moon things your freezer cranks out. Gross.
A spoon and a sharp knife.
Start by squirting a little agave nectar into the bottom of your glass. I don't like a lot... almost enough to cover the bottom.
Add a single maraschino cherry, with just a bit of the juice.
Smush the maraschino with the back of your spoon. No, I own a muddler. No, I do not muddle my Old Fashioneds. Frankly, I don't care for bits of smashed-up fruit at the bottom of my drink. I just crush the cherry to release the juice. Also, since I'm using agave instead of sugar, I don't have to worry so much about dissolving anything in the cherry juice.
Add the bitters. As I said above, I use both aromatic and orange. Four or five nice drops of each. Your tastes may vary.
Add the bourbon. I use two ounces... or so. I went a tad over this time, as you can see in the photo. So be it. Don't go crazy. If you want or need to drink four ounces of bourbon, have two drinks.
Stir. Stir well to mix the juice, bourbon, bitters and agave nectar.
Add enough ice to almost fill the glass. As I said above, use good ice! I do have molds to make giant cubes (balls, actually), but I actually like the mellowing effect of using good cubes in an Old Fashioned.
Cut a nice slice of peel from your orange, twist to release the juice, and garnish.