Purchase a set of iron-on hook-and-loop strips. Plug in your iron, allow it to heat up.
Place the first hook-and-loop strip on the top border of the sheer drapes. Remove the backing. Iron the strip so that it adheres to the curtain.
Remove the backing from the matching strip. Place it about 3 inches below the first strip. Iron the matching strip so that it adheres.
Pick up the drapes with the help of a friend. Place a curtain panel on the outside of the four-poster bed. Grab the top border and fold it over the top canopy rail so that the first hook-and-loop strip meets the second one. Continue draping the fabric and attaching it with hook-and-loop strips until the entire bed is covered.