Everyone does it differently. Sometimes you might even find yourself using different techniques. For example, sometimes I’m able to contemplate the mystery while saying the Hail Marys (usually when I have some free time and am not stressed). At other times I might say the Hail Marys then contemplate the mystery, or say the Hail Marys while contemplating an icon of the mystery, or I might read some commentaries or Scripture verses related to the mystery. And sometimes I may just say the Hail Marys quickly without concentrating much on the mysteries.
The thing about the Rosary is that it’s a very personal prayer, a communication between you and God, in which you should feel free to express yourself. Don’t ruin your Rosary experience by stressing about whether there’s a right way to pray it.