A couple of caveats with Lou's great instructions. I've discovered that plastic bags, the kind you get at the grocery store, are great to use with butyl. Plastic gloves will quickly grip and tear apart. Plastic bags aren't so fragile. They also store the leftover stuff very neatly... Well, neater than having it all over your hands and arms. Just turn the bags inside out when they become too messy to use and pull another over your hands.
Second. I'd steer away from silicone adhesive seals. Once it touches anything like paint, even after cleaning it off as best as you can, you'll find that years later, it makes a mess of your finish and the area that you tried to repair. If you have any gaps left over, it's probably better to go back with small bits of butyl and a dental type tool to pack it into the openings. Butyl cleans up fairly well with acetone.
My 2ยข