Good putting is a lot more mental than physical. Not a lot can go wrong with your stroke on a six-footer—it's a fairly short and quiet motion.
A six-footer is by no means a gimme, but it's still short enough that it stings when it doesn't go in. To make more of these, start by locking in your speed. It's the most important part of every putt. And when you assess speed, don't just factor how fast the ball needs to roll to get to the front of the cup. Think about it: You're not trying to be so precise with your putting that the ball falls in on its last rotation. So forget the front of the cup. You should be looking at a spot 1½ feet beyond the hole. You'll still be in tap-in range if you miss, but now you know the ball is going to get there every time.
Once you've determined that spot, then you can read the break. Start by walking to the hole, and try to picture the line in your head, keeping in mind that it continues 18 inches past the cup.