You can customize a special practice on your trouble keys to fine-tune your skills. According to your taget, GS Typing Tutor will generate the trouble keys report for you, which is known as "Worst and Best Keys".

On the main-screen, click the icon "Special Practice" into the section. You then select the practice characters, it can be the "Worst keys" or you specify directly the characters, you then click the button "Go..." to begin.