You can assign an existing label to an email message in several ways:

• Click and drag the message to the desired label in the left-hand navigation menu.



• Open the message, then press the Labels button in the top menu bar. Check the desired label(s) from the drop-down list and press Apply.
  
  NOTE: You may also create a new label from this drop-down menu by pressing Create new at the bottom of the Label menu.

For more information about Gmail labels, see Organize your Gmail inbox using labels on the Gmail help site.