You'll need to earn 23 medals in the game to earn this achievement. You can earn one medal for each type of exercise, and you'll earn 1/8 of a medal as progress each time you complete one session of an exercise. Progress can be tracked on the tile for each type of exercise, just to the right of the name.
Since you need to complete all exercises to earn Meritorious, it's best to work toward this at the same time. Some exercises do not have eight different versions available, so it will be necessary to repeat some exercises to complete those medals.