Measure the wall height next to the exposed studs.
Roll out a strip of insulation on the floor, making it longer than the height of the wall.
Lay the tape measure next to the unrolled strip. Lay a square across the width of the insulation strip at the wall height measurement. Cut the insulation strip to that size with your utility knife.
Press the insulation strip into the wall, between the two first studs, with the paper side facing out into the room. The insulation strip should completely fill the space between the studs.
Shoot staples through the sides of the paper into the sides of the studs, holding it in place.
Repeat the process to fill each space between each pair of studs. Use the utility knife to cut around any electric boxes and other obstructions. Drywall will go right over the studs to cover the insulation.