The video supercut:
This 1916 Caquot observation balloon was made for a WWI centennial meetup of model airplane enthusiasts. Its creation is extensively documented on the RC-Groups forum.
Bowman813 made a great instructional covering the use of a hand iron and cardboard forms to seal plastic sheeting. He used a transparent thermoplastic polyurethane sheet (PU), but the techniques could be used on more common sheeting made of polyethylene or polypropylene ((HDPE, LDPE or PP).
The silver and black paint is good choice for its light-blocking properties on the transparent film. The Caquot balloons of 1916 would have been green or tan for camouflage, but I doubt this balloon is big enough to support two coats of paint and a second color. A single coat of tan or green would have glowed in the flight. Silver has the right silhouette in the sky, especially since all our memories of WWI are second-hand and black & white.