Measure the wall gap from the bottom plate to the top plate. Lay a piece of insulation on a flat work surface. Transfer the measurement you took to the insulation and make a small mark with a utility knife.
Lay a level on the insulation, lining up the level's edge with the utility knife mark you made in Step 1. Press down on the level to compact the insulation. Cut through the insulation with a utility knife, using the level as a straight edge.
Insert the insulation into the wall cavity between the wall studs. If your insulation has a paper vapor barrier, open the barrier's side flanges and lay them over the wall studs. Staple the flanges' upper portion to the wall studs.
Split the insulation in half from the bottom up. Push one half behind the electrical wires and lay the other half over the top of them. If your insulation has a paper vapor barrier, lay the side flanges over the wall studs and finish stapling them in place all the way down to the bottom plate.