Measure the height of the wall, alongside one of the studs.
Put on cloth gloves. Stretch out a strip of insulation over the floor. Lay the tape measure next to it. Pull out the tape to the measurement you took of the wall.
Set your square on top of the insulation strip at the measurement. Cut the strip, using the square to make the cut straight.
Put the cut insulation against the wall, between the first two studs. It should fit tightly enough to stay there. Position it so the paper side faces out (toward you).
Secure the piece by shooting staples through the edges of the paper and into the sides of the studs 12 or 16 inches.
Use your utility knife to cut the insulation around any electrical fixtures on the wall. Repeat for each space between each set of studs. The wall is now ready for drywall.