Make two marks on the ceiling above the foot of the bed, even with either side of the bed. Make two marks on the ceiling 12 inches down from the head of the bed, also even with the sides of the bed. Make two marks on the wall that are even with the side of the bed and about 2 feet above the bed head. These are the positions for attaching the curtain rods which will hold the canopy.
Position the curtain rod mounts on the marks on the ceiling and wall. Attach the curtain rods in position using toggle bolts; drill through the fastener holes of the mounts.
Hold the end of a piece of string on the bed frame at the head, just below where the mattress starts. Drape the string over the curtain rod on the wall and the two on the ceiling, imitating the way you want the material of the canopy to hang; there should be around 2 feet hanging straight down at the end of the bed for the end of the canopy. Once happy with the drape effect, cut the string and measure it to get the required length of material needed for the canopy.
Cut out the canopy dimensions on two pieces of fabric, adding an inch all around for seam allowance. For example, if the canopy is to be 3 feet wide by 8 feet long, then cut the fabric to 3 feet 2 inches by 8 feet 2 inches.
Lay the two pieces of fabric one atop the other, right sides together, and sew together on a sewing machine using a 1-inch seam allowance and leaving a gap of 5 inches unstitched at one end.
Turn the canopy in the right way through gap you left unstitched, then top-stitch the gap closed. Press the canopy flat with an iron.
Drape the canopy over the curtain rod on the wall, then the curtain rod that is near the head of the bed, and finally the rod at the foot of the bed to finish the temporary canopy for your bed.