Why when scuba diving why do they equalise their ears and how do they do it?

Answer 1

There are several, already answered questions on this subject.

Scuba-divers have to equilize their ears during decent, because the middle ear doesnt automatically pressurize (unlike other body parts, like sinuses).

This is done by covering/squeezing ones nose using the thumb and index finger, and gently blowing air out your nose. The air, not being able to escape through the nose, will be redirected to the middle ear, thereby pressurizing it.