Good to hear from you. That’s a large barrel! The most important element is prepping the barrel. Start by filling it with warm water and let sit for 1 week. Empty it, and then rinse with warm water. Using a metal funnel, fill the barrel with 3 liters of the cheapest white liquor you can find (grain alcohol is fine, higher the ABV the better) and let it sit for 2-3 months.
Once you are ready to make the cocktail, empty the barrel. You’ll want to reserve the grain alcohol for use in cocktails (it’s going to be a chestnut brown by the time you take it out).
You’ll need two 750 bottles of rye –the Canadian Club 100% rye is in the PLCB shops for under $20 and it’s perfect for this– and one 750 bottle of Dolin red vermouth. Empty them into the barrel. Top off with some of the grain alcohol. Don’t use any more than 1/2 cup of the grain alcohol, though (it’ll overpower the cocktail in too high a percentage). Finally, add about 1 teaspoon of bitters.
Let sit for a week, and it’ll be ready to go! This will keep drinking well for at least 6 months. Enjoy!