This achievement requires that you finish a hole exactly on par 50 times. Finishing better than par does not count toward the achievement.
If you're patient, this should come naturally as you play the various courses in the game, especially if you're going for the various achievements for finishing each course on par. However, if you want to knock this out sooner and more easily, you can replay the Forest course in Classic mode. Most of the holes on this course can be easily finished on par. This is especially true for the first ten or so, the majority of which are par 2. Some of the early ones are actually so easy to hole-in-one that you'll need to adjust your putt slightly so you don't come in under par.
Progress toward the achievement counts immediately after you finish a hole on par and the achievement will unlock as soon as you've finished the 50th par. You do not need to finish the entire 18-hole course to keep your progress.