I've done this before simply by taking two ziplock/sandwich bags, one inside the other.
I take both bags, inflate them slightly by blowing into them, seal them tightly, then gently push on them to make sure they seal correctly. I put my phone (in my case my tablet, yes I know) in the first one, suck out as much air as I can get, seal it (very well!), then take the second one, put the first bag inside the second bag, suck all the air out, and seal it tightly.
Sucking the air out, in my experience, helps with the seal between the lens and the bag - if there are bubbles, your video will be absolute rubbish and hard to watch. In my experience, a tight seal between the bags and the camera lets me get clear shots.
Note that if you drop your phone, it will sink - your phone is heavier than water, and there isn't any air in the bags to provide flotation.