How Do You Count Cards?
Here's a basic plus-minus count.
Keep a number in your head. That number is called the running count, and it starts at zero whenever the deck is shuffled.
As cards are played, every time you see a card with a value of 2, 3, 4, 5 or 6, add one to the count.
Every time you see a card with a value of 10, J, Q, K, or A, subtract one from the count.
When the count gets larger ("a positive count"), the odds have swung a bit in your favor.
When the count gets smaller ("a negative count", a count that's less than zero), the odds have swung a bit in the casino's favor.
Take the running count and divide it by the number of decks remaining to be played to get the true count. This is what you base your play and betting decisions on. Here's a sample betting strategy: "If the true count (TC) is less than 2, bet one unit. If TC is 2 or greater, bet (TC-1) units."
You will also vary your play based on the count. For instance, if the count is positive you will stand on a 16 versus a dealer 10 showing (or on a 12 versus 4 showing); if it's negative, you'll hit those two hands. If the count is +3 or above you'll take insurance, whereas basic strategy says you normally wouldn't take insurance. There are many index-based strategy variations you will want to learn; 16v10 and insurance are only the biggest two.
The bottom line here is that you're not having to keep track of very much as the cards go by. The index numbers, basic strategy, and your betting strategy are things you memorize well in advance of sitting down to play. As you play, you only really need to keep track of a single number - the Running Count. It goes up, it goes down, and sometimes you convert it to a true count and make decisions based on it.
This is why playing to a 6-deck shoe isn't hugely different than playing single deck. The same skills are used. The strategies employed are different, of course, but the essence of the game is still the same.