What You'll Need
Insulation blower
Insulation
Mask
Rag
Plastic sealer
Staple gun
Step 1 - Dust
Use the rag to dust out the area before you fill it with insulation. Carefully go in and out of each opening to ensure that there is nothing left.
Step 2 - Blow Insulation
Put the end of the blower into the opening. Carefully fill the opening with insulation. Typically, you will be able to fill most of the space with one or two blows. However, if there is empty space in some openings, insert the blower into those and fill individually.
Step 3 - Apply Plastic Sealer
Use a staple gun to cover the floor truss with a plastic sealer. You can find some at many hardware stores that can be cut to size. Staple around the corners to keep it down.
Source: www.DoItYourself.com