If the water has entered the cabin,  cover your mouth and nose before securing the device to your child. Open it by tearing in the direction of the arrows. 
Do not worry about your personal belongings or appearance by inserting your head through the appropriate hole. If the vest malfunctions, there is plastic tubing that will direct your breath to inflate.
Holding it tightly to your chest, do not attempt to jump immediately into the water.  It may or may not be present at the time of the emergency landing (see diagram). 
If you are under water, wait for the lights to point you in the right direction. Do not attempt to re-board the plane from this angle.
STORY OF A DESCENT Part One