Here's a video from 3Jack Top-25 Putting Instructor, John Graham.
To me, being able to determine the stimp using AimPoint is the fundamental aspect of the green reading system.
First, you have to determine the % of slope.
Then you determine where the ball is with relation to the slope.
Then you have to be be able to aim the putter correctly, hit the ball where you have aimed and do it with the right amount of speed to correctly determine the stimp.
It's like starting off practice on the driving range with a few half-wedge swings to get warmed up. It's very simple and very basic, but if you cannot properly determine the stimp using this method, then you are not going to perform AimPoint very well.
What I usually do is once I think I have the stimp from 5 feet away, I then putt a few from 10-feet away to make sure that I have the stimp correct. Plus, the same fundamentals apply in being able to read the % of slope, where the ball is with relation to the slope, aim, hitting it where you aim and hitting it the righ amount of speed.
What's nice is that if you practice it for a while, you start to get to the point where you can generally tell what the stimp is by just looking at length of the grass on the green.
But, understanding how to find the stimp is more than getting your AimPoint chart set correctly, it's about being able to execute the basic principles of AimPoint.