There are millions of cards - so having some kind of a plan as to what you're looking for is the best bet. I have a list of players and I just go down the list plugging their name into the search and buying stuff I like that is fairly priced for the first day or two. After that I generally start looking for stuff to flip or reload my port with - star/rookies autographs, graded stuff, HOF relics/autos, etc.
Would probably be a good idea to create an account ahead of time and do some research today and tomorrow regarding how it works. You do need to load funds into your account to make immediate purchases or you can load items into your cart and check out when you hit 25 items - you get the shipping discount every time so you can just check out every time you hit 25 cards so you don't miss out.
It usually starts out pretty fast and furious and then seems to go in waves throughout the weekend for me - but this is the most cards I've ever had and the best discount in a long time so I expect pretty steady sales as my stuff goes well even without a discount.
If you have any specific questions - shoot me a PM and I can try and help out. I've got about eight years of experience as a buyer and six years as a seller.
I've never done Black Friday on COMC. How does it work? Do you just have to find the bargains or is there a trick to it?