After you board and you go check out your cabin for the first time. Find where the life jackets are. Usually in the closet or under the sofa in a drawer. If you need a longer strap let your steward know he will replace it.
Find out when Muster Drill is. About an hour before the drill go get your life jacket and go to your muster station. Everyone is required to do the drill.
They disable the elevators during the drill. That's why I say go early. So you can use the elevators. Hopefully your Muster Drill Station will be in a lounge. Bring a bottle of water and you can be seated and comfy. If it's on deck wait in a near by seating area.
Don't go to the station untill you have to. Otherwise you will be standing in the back of the crowded deck. Bring a bottle of water with you.
I almost passed out from the heat once during a Muster Drill. It was soooo hot and the life jacket made it hotter... Oh and you don't need to put it on until instructed to. They just need to know you can put it on and buckle it up.
Hope it helps.
Hugs and Happy cruising,