Measure one of the wall studs from the top of the open wall to the floor plate.
Stretch out a course of pink batt insulation over the floor. Set a tape measure next to it, stretched out to the measurement you took of the stud.
Lay a carpenter's square on top of the insulation strip, at the measurement as indicated on the tape measure. Cut the strip with a utility knife along the edge of the square. Makes sure to cut all the way through the fiberglass and the paper backing.
Hold the cut insulation strip into the wall space between the first two studs, so it fills the space. Position it so the paper side faces into the room.
Staple the piece in place with a stapler, shooting a staple into the sides of the studs every 12 inches.
Use the utility knife to cut away the insulation from around any electrical fixtures on the wall.
Fill all the spaces between all the studs in the same manner. Once all the spaces are filled, the wall is ready to be drywalled.