Incredibly easy if you don't actually want to do the workout.
Load up the game, and select any workout. Select "skip" on the intro, then once the workout starts, push the menu button on your controller. This will bring up a pause menu, and select the fast forward option with two triangles and a line as the icon. This skips the workout and brings you to the last 20 seconds of the video. Go through all the prompts and it will count. Then rinse repeat with the same workout video. Should take you 5 minutes or so.
You get 10 stamps, but you need to complete a workout after you get the achievement in order for them to be added to your stamp card.