Put a tee in the back of the cup and hit it squarely with every putt
Although his swing looks a little loopy Jim Furyk is a great example of a one plane swing, he takes the club outside coming back but his delivery position is exactly on one plane.
A two plane swing manipulates the shaft on plane by rotating the club. The one plane swing is more athletic and naturally places the club on the correct swing and shoulder path, which is better for women and requires less maintenance.