One of my favorite drills to get a player hitting the ball squarely is to make a simple training gate using a pair of tees.
Set one tee just inside the heel of the club at address, and another just outside the toe (above). The tees should allow enough space for the clubhead to swing through. I'm doing it with an iron here, but you can use any club—even a putter.

If you swing an iron through the gate and make a consistent divot, you're guaranteed to hit the ball in the center of the clubface. The tees will let you know if you make a mistake. If you're hitting the ball toward the toe, you'll knock down the inside tee. If you're making contact near the heel, you'll hit the outside tee. It's powerful feedback.
Start with slow swings and no ball, then work your way to half shots with a ball, then to full-speed swings. You'll be puring it in no time.
How I See It
Illustration: Mark Matcho

HOW I SEE IT

The phrase "great ball-striker" is code on the PGA Tour for somebody with a sharp iron game. But I think the true greats with the irons have specific areas where they've excelled. It starts with greens in regulation (GIR). When they were dominating, Jack Nicklaus and Tiger Woods were always near the top in GIRs. To look at it another way, the players who lead in average proximity to the hole from 50 to 125 yards are giving themselves more makable putts for birdie. Last year, Steve Stricker led that stat at 14 feet, eight inches. That's how a short hitter—and great putter—wins $3.5 million in only 19 tournaments.