Click the View action button of the selected agent from the agents list page
OR go to your profile page, in case you want to add a note for yourself. To do this, click Menu > My Settings > My Profile
In the view agent page, look for the Notes tab. Click the Notes tab button.
Under the Notes tab, click the Add New button. You can also directly click the Notes icon located at the upper right corner of the agent's view page.
Then, enter the note information in the note fields as shown in the figure below.
Once everything is entered, click the Save button.
The new note will then be included in the list of notes under the agent's Notes tab.