You just keep a running tally of what cards are left in the deck. Certain cards are -1, some are 0, some are +1. When the deck becomes very positive, it is in the players odds that they will win a hand and they should start betting more heavily.
There was a movie 21 that a group of MIT did this. They would have a group play together just to make it not obvious that one person was situationally betting heavily when the deck went positive, which looks like card counting to a house..
This is a really good story about a church group that went to casino's to raise money for their church by county cards. Listen to them describe it and it is very much just like a boring job.
https://www.thisamericanlife.org/466/blackjack