Instructions:
For the Tile drill, you need to use either the lines on a tiled floor or the lines on hardwood.
To start, simply set up over a line on the floor with the line centered with the center or sweet spot of your putter.
Next, if you are a player that swings the putter head on a straight path, swing the putter head back and forth watching it with your eyes to make sure that the center of the putter stays over the grout line on the tile to make a perfect straight path. Your putter face should also stay square to the line throughout the stroke.
If you swing your putter more on an arc, use the line and watch your putter head as it arcs slightly away from the grout line on the backswing and again on the follow-through. The face of your putter should stay square to the arc path, not to the grout line. This means that you will see a slight opening of the face on the backswing and a slight closing on the follow-through.
Practicing with a line repeatedly indoors will help you train your eyes to see a consistent path and help train your hands, arms, and shoulders to feel one.