Measure the distance from one end of the attic to the other across the rafters.
Cut a piece of silver foil material to the determined length with a utility knife.
Hold one edge of the insulation piece up to the roof near the ridge so that the silver part is facing into the attic. Have a helper take the other end of the material to the other side of the attic. Staple one end of the foil to a rafter, then repeat at the other end. Staple to rafters in between the two ends.
Repeat the procedure until the entire roof area is covered. Leave a gap from the bottom of the last row of foil material to the edge of the roof so your house can breathe. If your roof line stays the same, you will can use the measurement from the first section for the rest. When you install foil material on the the other side of the roof ridge, be sure to measure again. The measurements will usually be the same, but not always.