Put in 5 lbs. of sugar. I like the Hires extract, dump that in. I add about 1/4 c. of vanilla extract at this point too. Put in about a gallon or so of water, then stir really well to dissolve the sugar. Add more water until the mixture is up to the 4 gallon mark. I use about 5 to 6 lbs. of dry ice, but break that up into chunks about baseball size. Over the course of an hour, add the chunks one at a time, and each time after adding a chunk, put the lid on to help keep some of the carbonation in. Be careful, though: you need to hear some air escaping. If it's totally sealed, your root beer will blow up.

After an hour it should be cold enough (and semi-carbonated enough) to serve.