Unfortunately all of the menu navigation in the game must be done with the uDraw tablet, though you can use your finger as the pen by mimicking its function, by moving your finger around and then lightly pressing where you want to select and releasing your finger from the screen immediately after.
From the main menu select 'Customize' and then Clue Creator
From here press 'Edit'. This will bring up the 360 text editor, simply input any character and press
I went for inputting 1. This saves the clue and you are ready to press the little arrow below where the clue is. Pressing right will go to the new clue for you to edit and repeat the process.
Once you've created number 20 the achievement will pop.