Instructions
Download and install a threat meter add-on.
Open the threat meter's configuration window by pressing the "Esc" key, clicking on "Interface," then selecting the "AddOns" tab and clicking the name of your chosen threat meter add-on in the list.
Adjust the threat meter's configuration to suit your purposes. For example, if you regularly play as a damage dealer, you can configure the add-on to make your screen flash or play a warning sound when you are about to overtake the tank on the threat list.
Move the threat meter's window to a location on your screen where you can easily see it but it will not obscure combat action.
Pay attention to any warnings you may have set up and, more in general, to your position on the threat list during combat.