To make a jet pack for a costume, duct tape two 2-liter bottles side-by-side. Tuck red and orange tissue paper in the open ends of the bottle as flames. Tape ribbons to both sides to create straps that go over the shoulders.
Before assembling the main structure of the jet pack, spray paint the bottles silver to make them look like a real jet pack. Let the paint dry completely before continuing. Place the bottles side by side so they line up exactly. Place a piece of tape horizontally across the bottles. Continue tape around the bottles to hold them securely together.
When creating the tissue paper flames, gather several sheets of different colors together and fluff them out to make them look like spread-out flames. Tuck the ends into the bottle openings with a little glue to keep them secure.
Cut the lengths of ribbon long enough to loop around the shoulders to make straps. Turn the taped-together bottles upside down so the open ends face down. Tape the ribbons in place, securing one end near the top and one end near the bottom with slack to make a loop.
Add decorations as desired to the jet pack. Colored or patterned duct tape works well to add stripes or other decorations. You can also paint designs on the finished jet pack.