This article isn't about how to count cards in a casino setting; it's about how to practice and learn to count cards before you attack the tables. You can read about how to count cards in a casino here.
If you want to learn how to count cards, you need to practice, and you need to practice often and consistently. Repetition is the mother of learning.
The first thing you need to do is to pick out a card counting system to use. I recommend starting with the Hi-Lo Count, since it's generally one of the easiest card counting strategies to learn. Once you've learned that one, you can move on and learn different strategies.
You'll need a deck of cards to practice with. When learning the Hi-Lo Count, you should be able to deal through the deck, one card at a time, and end up with a total of 0 when you finish. Start slow, take your time, and make sure that you're accurate.
Once you've managed to count through a deck of cards one card at a time, try practicing by dealing two cards at a time. This will increase your speed automatically, because many card combinations will cancel each other out. (Since all the cards are +1 and -1 or 0, this will happen pretty often.)
Next, try dealing 3 and 4 card combinations.
When you've got total accuracy by counting cards with 3 and 4 card combinations, you're ready to start doing two things:
Most professional card counters can count through an entire deck in 30 seconds or less. You should be 100% accurate in 40 seconds or less before you try counting in the casino.
This might sound really tough, but think about it this way: when you're playing in a casino, you'll have multiple distractions, and you'll also need to be so proficient at counting that you can do it without actually looking like you're counting cards.
Set yourself a time every day to practice, and stick with your schedule, and keep at it until you've got it. Then practice counting a bit at the low stakes tables until you're confident you can start winning some money.