Measure the length of the joist from wall to wall. Cut a sheet of fabric to this length.
Measure each side of the joist from where it meets the ceiling to the bottom of the joist (the height of the joist). Add each side's measurements together. Measure the width of the joist and add this measurement to the two-side total.
Cut the sheet of fabric from Step 1 to the width of the sum from Step 2. Line up the edges of the fabric so the sheet wraps around the joist and the ends line up flush where the joist meets the ceiling.
Staple the fabric in place using heavy wood staples. Be sure to staple where the fabric meets the ceiling, on the bottom of the joist, and on the sides of the joist.
Repeat these steps for each additional joist.