Remove the screws from the face of the electric panel with a screwdriver. Lift the face off of the panel to expose the interior wiring inside the breaker box. Verify that the electricity to the box is off before continuing. If you have not already done so, use a wrench to loosen the lugs in the main breaker and remove the large electric cables coming from the meter box that are secured under the lugs.
Pull the electric cables up and out of the box. Repeat the process for the ground cable that is attached to the lug on the neutral bus. Pull the cables coming from the lugs in the center of the transfer switch box into the house panel. You should have two hot wires and one ground wire.
Push the ends of the hot wires under the lugs on the main breaker. Insert the ground cable into the lug on the neutral bus. Use the wrench to tighten the lugs until the cables are firmly attached to the breaker box. Lift the cover for the breaker panel back into position, and use the screwdriver to replace the screws to fasten to the box.