Download TeamViewer to your computer (Microsoft, Apple eller Linux Microsoft) by clicking the link above.
On PC select Run, on Mac open the dmg file (double-click on it in the Downloads folder) and start TeamViewer in the shown virtual disk.
TeamViewer now displays an ID and a password. Please, inform the password to the IT supporter over the phone. (The ID is 9 digits and the password is 6 characters. If not, you have an old version of TeamViewer. Please, download the newest version by clicking the link above.)
Awaiting authentication is then displayed down in the TeamViewer window and later, after the IT supporter log in, that window "jumps" up into the corner.
On the screen, you will now be able to see how your computer is used by the IT supporter. Please don't touch the keyboard and mouse, unless the supporter asks you to enter e.g. a password.
Finally, the IT supporter quits the TeamViewer connection, and this closes the TeamViewer program as well (otherwise just quit the program by yourself).