Layout your materials on a large sheet cardboard.
Pull out approximately 6 feet of the painter's plastic and cut it forming a sheet 9 x 6 ft. It's important to have plastic that is 1/2 mil (12 microns) or thinner. Thicker plastic is too heavy to fly.
Fold the sheet in half along the 9 ft side forming a two-ply sheet 4.5 x 6 ft. Use a hot knife to seal the two 4.5 ft edges. An electric hot knife makes this really easy, but it can also be done with an old metal knife heated over a flame.
Check your edges and correct any large holes. A few small holes won't cause problems.