You just lay the "bladder" in your tub, put the sleeve over the faucet, and fill it up.
When you need to get water out of it, just use the accompanying siphon.
Here's a little video about it:
I did some additional research and felt the benefits were well worth the small investment, so I ordered it. It just arrived yesterday and I'm ready to go put it in the bathroom cupboard so it's right there when/if we need it. I ordered it from CheaperThanDirt.com and it was $29.99 plus shipping of about eight bucks. Not bad for a container to hold 100 gallons of water in case there isn't anymore for some reason!