Before even finding them a beach, Shell Bell gives Juliet a rundown on how to control the wand:

"You're in control of a bunch of things. Shape, location, temperature, how fast it consumes available fuel - it doesn't need fuel at all, you could have a floating fireball in a sealed chamber and it wouldn't take your oxygen if you didn't want it to let alone die for lack of stuff to oxidize, but it'll take anything you do want to feed it. If you lose attention on any of those things, the fire will be out of control on all counts and go on according to the laws of physics. You can get it back as long as it's contiguous. But you can't control more than one piece of fire at a time, so until you really have the hang of it and never lose your fire before you're ready, don't let it spread too far where it'll burn out intermediate patches."