In a perfect world, putting on a straight back and straight through type of path would be ideal.  Unfortunately, executing a straight back and straight through stroke, every single time, is virtually impossible (especially in windy conditions).  It's kinda like bowling a 300 every single time.

The golfer needs to understand we stroke the putter on a SLIGHT arc.  We don't want a ton of opening and closing of the putter face during the stroke, however, slight opening and closing has been found to be the most consistent way to deliver the putter's sweet spot into the golf ball.  Which means more made putts for you!

So how does one practice putting on a slight arc? Glad you asked. 

With the MS-3D Putting Arc Training Aid, you are now able to not only visually see the path of a slight arc'd stroke but also you can practice it.  Repetition is key.