The correct way to don a life jacket is as follows (According to Solas chapter III):
First tie the top straps with a reef knot.
Then tie the straps around your waist, also with a reef knot
When jumping into the water from a height (life jackets are rated for 4.5m) there is always a risk of the life jacket coming loose, this is very dangerous because it can break your neck, or even leave you flaoting face down in the water.
Correctly tied your life jacket should turn you right way up oin any conditions within 5 seconds .
When jumping you should have one hand over your nose diagonally across and the other diagonally over that arm and holding onto the life jacket.