If you haven't already come across the Floating Lanterns by Wishlantern you're in for a treat.
These floating water lanterns come flat packed but are very easy to assemble. Simply put it's just a case of pushing the 4 dowel rods into the balsa base and then sliding the rice paper cover over the rods. They come with tealights and to light the lantern you slide the rice paper cover up and then light the tealight before sliding the rice paper cover back down.
Check out the images and video's below for a visual guide and if you'd like to get some for your next party they are available here