el3n1:You might consider researching and learning some green reading techniques,
My method:
Work out the distance you need to hit, then survey the line using both putt and chip mode from behind and in front of the ball.
After you have done this, you need to visualise the speed the ball will travel over the distance to the hole at the distance hit ie: how long it will take your putt to roll 1-2ft past the hole.
A 20 ft putt downhill 8 inches on a 13s green you'll only need to hit around 7-7.5 ft, whereas the same putt on a 7s green you'll need to hit about 17 ft.
That's a big difference for the amount of time the ball will travel over the same distance of 20ft.
Importantly the more time a putt takes to roll over a given distance then the more it will break and vice versa.
So, once you've got the math sorted it is only a matter of, and I'll quote Chevy Chase here, being the ball.
Be the ball ;-)