How do you count cards successfully?

There are numerous ways to count cards, the easiest is to simply apply a value of +1 or -1 for various cards. For example +1 for all "10's" and -1 for the cards A to 4. This then allows you to keep a count and determine if the deck is rich in 10s or low cards, and then to apply your play to that style.

Keep in mind you will need a good betting strategy as well as understand that you can't just play what "the book" tells you to do. If you simply play by "the book" and keep the same bet, then the house keeps it's 4% and slowly whittles you down.