In order to perform a Redo, it is necessary to have performed an Undo. So, perform an action to undo such as erasing a word.
Now that the text has been erased, undo this action by using the shortcut (Command/Ctrl + Z) or the menu.
Your text will reappear as undo was performed.
Now, to Redo your last Undo, use the keyboard shortcut of Command/Ctrl + Shift + Z. Your Undo was reversed and your text has been erased.