Step 4: Load it up
Why? If you make your whole day depend on one single ziploc bag, there's a very real possibility that you'll wind up with a cooler dripping bourbon out on the ground when you open it up, and friends that don't want to talk to you ever again.
If you divide your supply into quarts, and one of the quarts leak, then it's still in the gallon bag. It also makes it easier to shift the liquor around to get the best fit.
If you're attempting this with beer... ugh... well first, why are you drinking beer like a little girl? Whiskey is for men with hairy chests, and it's refreshing on a hot day with some ice, which you need for the cooler, remember?
Second, if you think you're going to do this with beer, you have to do everything from this step onwards within about an hour of expecting to open the bags again. And at that, I'm just guessing. Chances are beer will end in diaster. Just sayin'.