The first step to putting is being able to read the green. This is a fairly simple process, when you go to putt you will see a grid on the green. If the dots on the lines in between intersections are moving the green is sloped towards the direction the dots are moving and the ball will drift that way after being hit. How fast the dots move is how sloped the green is.
Next you want to choose how much power and at what angle you want to hit the ball. if the green is sloped left you'll want to hit it right and vice versa. If the green is sloped towards you, you'll need to hit it harder to get up the hill. It takes practice but soon you'll be putting with the best!