They can type just their username . Find out if they know how to use a mouse. If not explain, rest hand on the mouse to hold it steady. A mousepad will help. Talk about clicking.
Single click means click one time using the index finger on the left button.
Double click means clicking twice on the left button. Any menu item that has a small black arrow next to it indicates, that there is a submenu. Click on the menu item to display the submenu. Displays the time. Point out the Menu Bar and how to use it.