Dan, this isn't going to be the most helpful answer, but here goes:
I originally made these with Kirschwasser (which is a cherry brandy, you can get it at Binny's, among other places). Later I started making some with whiskey because I usually use the cherries in whiskey drinks.
Last year I ran out of the kirschwasser and whiskey and still had some cherries. I realized I had a big bottle of vodka that I never use, so I made some with that. I also tend to add spices to the mix (esp when I used vodka). I can report that the cherries were great no matter which alcohol I used. You can detect differences in the taste across all three types of alcohol, but all three work well in Manhattans. My advice is to buy a mid-priced bottle (Benchmark whiskey, for example) of whichever you feel like using.
You can also use Luxardo Maraschino liquor, which is what maraschino cherries were originally packed (and why they are now called Maraschino cherries). But this stuff is a bit too pricey for this application, I think.
Syrup. Last year was the first year that I used sugar or syrup. If you have "bar sugar," aka "superfine sugar," you can use that instead of making syrup. Superfine sugar is available at Jewel (this might be the only think that I buy at Jewel, frankly), but isn't at most smaller grocery stores or Whole Foods, as far as I know. Superfine sugar is basically sugar that is ground finer than regular sugar. Unlike regular sugar, it dissolves in cold liquids quite easily. It is therefore good to use in cocktails and in making brines. Anyways, you can make a simple syrup or use Superfine sugar in the cherries.
Here is the least helpful part of this post: I don't remember how much sugar/syrup I used. I also don't remember if I used syrup or Superfine sugar! My recollection is that I used a couple of tablespoons of syrup or sugar per quart-sized mason jar. I don't think the exact amount is particularly magical - they will just be a little more or less sweet. You can always make one batch sweeter than the other to experiment.
I just consulted two recipes that may be more helpful: both recommend 1/4 cup of sugar per 2 cups of alcohol. (If you use syrup, you'd use a little more than 1/4 cup of syrup per 2 cups of alcohol.) The thing is you won't know exactly how much alcohol you need until you pack the jars with cherries.
This year I will do the following:
1. Pack mason jars with rinsed (but not pitted) sour cherries.
2. Add to each jar about 2 tablespoons of Superfine sugar or about a 1/4 cup of syrup. Add a mixture of spices to each jar (clove, cinnamon, etc.)
3. Fill each jar with alcohol.
4. Seal the jars, shake lightly, and store in the fridge. Don't use for at least a week or so, but give them a shake every few days during the first week just to make sure everything is mixed.
You will notice a big difference between my version and Ronnie's: he heats the brandy and then processed the jars in a hot water bath. I've tried this and find that the texture of the cherries deteriorates faster than leaving everything cold. Note that if you leave everything cold, you need to store the cherries in the fridge. They will keep for at least a year (at which point fresh cherries will be available to make a new batch).