To playtest the stats and achievements GUI, you can change the values that show on screen and make sure the correct values are shown on screen. Other than that, you can call each of the functions individually in a Start function to make sure that they work. To take this chapter a bit further, you can add stats to track as well as show achievements for them. You can also replace the text in the achievements with an image to better represent the achievements; this would be a nice finishing touch to them.