The below is a copy n paste from:
Team Natural Outdoors > Peck's Lake Spanish Mackerel
A bubble rig consists of a clear plastic bubble called a "cast-a-bubble" (it looks like a clear elongated bobber, but can be filled with water), a swivel, 3' of 50# fluorocarbon, a 1/0 treble hook, and a 3" long piece of a McDonald's straw. I'M NOT JOKING!
Slide your line through the clear plastic stem inside the Cast-a-bubble. Put your line in the small side and out the larger diameter of the stem. Tie a swivel onto the terminal end of the line. This will prevent the bubble from moving past the swivel onto the fluorocarbon. The stem pops out one side of the bubble allowing you to fill it with water. I usually fill it about 3/4 full. When the stem is pushed back through the center of the bubble it becomes water tight, trapping the fill water in place. If you've threaded your bubble correctly, the swivel will hold the stem in place rather than pushing it out and draining the bubble of water. Tie your 3' of flourocarbon to the swivel and thread your 3" long piece of McDonald's straw onto the leader. Tie the treble hook onto the leader with a clinch knot (or some other knot that allows the straw to move down onto the shaft of the treble). This will allow the straw to travel freely from treble to bubble.
The filled bubble will weigh at least 2 oz. . Using light tackle you can cast this thing a country mile. It is best to slow the descent of the bubble just before it hits the surface so that the straw and bubble don't tangle on impact (the straw will land on the far side of the bubble from the boat).