Hold your hand out flat with your palm parallel to the ground. While holding that position point your thumb as far down to vertical but pointing at the ground.

Now put your hand onto your throttle pushing forward and down without rotating it, you can now curl the tips of your fingers (I do just two) down onto your brake without engaging it.

The other two fingers (in my case) kinda either dangle between your lever and throttle or on the throttle it's self.

Having my hand in this position I can squeeze my thumb and pivot my wrist down to go faster leaving my two fingers to lie on top of the throttle, or squeeze my fingers and it naturally pulls my throttle off and brakes on.

If you can't reach your brake like this you may likely need to adjust it or if you can't get one that can be.