Start with a tall (almost full) water bottle, a clear plastic grocery bag, string, and scissors. You'll also need food coloring if you wish to have blue or greenish-colored water.
Cut out a square piece out of your bag, then pick up the center of the bag (which will be the jellyfish's "head").
Use a pipette to fill the head of the jellyfish with water, leaving room for air and keeping in mind that the head should be small enough that it fits through the mouth of your water bottle.
Gently nudge the jellyfish into the water bottle, and add food coloring plus more water to fill the bottle.