Measure the height of the exposed wall studs, from the floor to the ceiling plate.
Roll out a course of batt fiberglass insulation on the floor. Pull out the tape measure next to it, setting it to the length you need for the wall.
Lay a carpenter’s square over the insulation, alongside the tape measure, at the measurement. Cut the insulation off at that length, running your utility knife alongside the square.
Press the cut strip of insulation into the wall, between the first set of studs, with the paper-backed side facing out at you.
Use your power stapler to shoot staples to secure the insulation there, shooting staples through the paper backing along the edges and into the studs. Shoot the staples about every foot up both edges of the insulation.
Repeat for each pair of studs, filling all the spaces between them. Install your drywall over the insulation and onto the studs.