Here's a simple technique to improve your putting.

Before you can expect to putt well on greens with a slope, you need to learn to putt well on the flat.

This will train you to putt you ball through a straight line and hit the ball squarely on the face of the putter. (Remember every putt is straight)

Now you need to get a putter that has a line on it that you can see clearly, as this will be the line that you will aim at the center of the ball.

On the practice green (preferably one that you have at home, as this technique might not be appreciated) mark straight lines with a builders chalk line in several directions from the center of the hole spanning out around the green.

Now move around the green flicking the line to leave chalk marks along the ground. Next - place your golf ball on the line leaving some of the line on the ground visible behind the ball. This will help you to guide your swing. You will then be able to swing your putter using the line on the ground as a guide and the line on the putter itself to connect with the center of the ball.

In a very short time you should find that you are managing to putt from a relatively good distance and get the ball in the hole most of the time. This will help you to develop the correct posture and consistency to putt well.

After doing this for a while from different positions on the green, move the ball off the line and visualize that there is still a line on the ground by creating it in your mind, looking from the ball to the cup and back again several times.

Line up your putter as before and look at the line on your putter to hit the center of the ball. You are now retraining yourself to see an invisible line from the ball to the cup that you can use on any green you play on.