HOW was the Leaning Tower of Pisa stabilized?

The Leaning Tower of Pisa was stabilized with a massive engineering operation that took place in the 90s, in the attempt to stop the monument from falling. In fact, by the end of the 80s, the Tower was slowly heading toward its catastrophic collapse.
The Monument has been closed to the public for the entire duration of the works (over a decade: 1990 - 2001).
Here is how they stabilized it...