1. From WordPress Admin, click on Appearance -> Menus

    global click appearance menu

  2. Click on Screen Options from top right corner

    login menu screen options

  3. Check on CSS Classes

    login menu css classes

  4. Expand the Menu Item (you want to add CSS classes) and under the CSS Classes (optional) add your CSS classes as necessary. You can add more than one CSS classes separated by space.

    global menu add classes
  5. Click on Save Menu