It's easy (sort of)! Like people have mentioned, watch the movement of the lines on the green. the best tip though is to use the line on top of the meter to your advantage - it's there for a reason...
First, the flag location on top of the line is the exact location of the hole. If there was no slope at all, you would set your power exactly at the flag to get the proper distance.
Second, if the line slopes from the left -> down to the right you have an uphill putt so you should set the power meter a little past the red flag. If the line slopes from the right -> down to the left you have a downhill putt so you should set the power meter a little in front of the red flag.
Finally, and most importantly, is the slope of the line itself. As you position your shot from left to right you will notice the slope of the line will gradually change. This is a little hard to explain but after you take into consideration the movement of the lines on the green and get comfortable with how hard you should hit the ball, you should try to fine tune your putting line by making the slope of the line as uniform as possible from the start of the meter (where your ball is) to the red flag (where the cup is). If you make sure this line is as smooth as possible it will more than likely go in.
Here's a basic example... You are 30 ft from the hole and there is a slight slope to the left. You should move your putting line to the right to compensate for the slope. You then note that the line slopes from the left -> down to the right so you know it's an uphill putt so you know to time your press a little past the red flag. Next you look at the line itself and notice it slopes approximately 3 pixels. You should aim for te pixel transition to be every 10 ft on the line so it has a uniform slope.
I hope this helps but mainly practice makes perfect.