Measure the distance between the studs. This space has changed over the years so it depends on the age of your house. This measurement determines the width of the batting you will need to buy. Also measure the depth of the space between the walls. Different insulation has different thicknesses, depending on its R-value. You do not want to stuff a 6-inch thick batting into a 4-inch space as it will have to be compacted to fit, reducing its insulating properties.
Determine what R-value insulation your state building code requires. There is a chart on the Department of Energy's website that lists all the states and what the recommended insulation is for your area. Once you purchase the insulation, make sure you keep a copy of the label in your house paperwork files. According to the Federal Trade Commission, "the ... home insulation rule requires the seller of a new home to provide information on the type, thickness, and R-value of the insulation."
Dress appropriately. Wear a long-sleeved shirt with a button-up collar and cuffs, work gloves, a hat, a disposable dust mask and eye protection. Insulation fibers can irritate your skin and be toxic to your lungs.
To begin, measure the length of the space to be filled and cut the batting with a sharp utility knife on a wood surface according to that measurement. With the paper covering facing you, staple the insulation in place with a staple gun into the front edge of the stud.
Pack the insulation loosely into small areas but not around electrical outlets or switches that generate heat. These will need air circulation to prevent heat buildup. Electrical fire safety code usually requires a 3-inch space around these devices.