Determine the depth of blown-in insulation you will need. The depth will depend on where you live and the R-value of the insulation you use. Cellulose typically provides R-38 at a depth of 10 in.
You can determine the level of insulation needed by checking the map provided by the Department of Energy or get a specific recommendation using this Zip Code Insulation Calculator.
Use a tape measure and permanent marker to mark the required depth of blown-in insulation on vertical trusses around the attic. Be sure to measure from the drywall surface.
If you already have some insulation in the attic, determine its R-value and subtract that amount from the total desired to determine how much new insulation will be needed.