Once you've selected the plant that gets to endow it's life-giving liquid unto you, then wrap that bag around the greens, at least 3/4 of the way into the bag. Tie the top end off with some cord real tight so it won't fall. You'll also want to use a small rock to keep the corner low so the moisture will collect. Then you wait and let the sun do it's work. The plant inside will go through its magical process and within a few hours, SHAZAM! you've got water. The amount of water you collect will depend on a lot of factors like sun, cloud cover, type and size of plant, etc...but it should produce enough to keep you alive.
To collect the water you can open up the corner with the rock and drain it into a container. You might want to filter it because sometimes there can be debris or "other" things in there (free protein if you aren't opposed to eating creepy crawlies). You can also place a tube in the bag and use it as a straw if that's your thing. Anyway, that's a quick tip for survival. Make sure you put some clear plastic bags in your survival gear and you can get some water.