Use the switch as a template to mark the mounting holes. Hold the switch to the desired mounting area which should be in close proximity to both batteries. Mark the holes with a pencil and set the switch aside.
Drill through the pencil marks with a #12 drill bit inserted into a power drill.
Trace the positive battery cable on the engine’s starting battery to the starter solenoid. Loosen the positive battery cable’s retaining nut on the starter solenoid with a wrench. Lift the cable straight up from the solenoid. Loosen and remove the nut and washer from either position one or two (whichever you prefer) on the back side of the guest battery switch with a wrench. Place the end of the cable that you removed from the solenoid, with the flat part of the terminal facing downward, onto the battery switch post that the nut and washer were removed from. Replace the washer and nut on the switch and tighten it to 150 inch-pounds with a torque wrench.
Cut a length of 4/0 size battery cable that will be long enough to reach the starter solenoid from the switch with a pair of tin snips. Attach cable lugs with a ½-inch diameter hole to each end of the cable, using the instructions that came with the lugs. Attach one end of the cable to the starter solenoid and the other end to the feeder terminal on the backside of the battery switch with the original nuts and washers.
Trace the positive battery cable on the remaining battery to the accessory load position. Remove the cable and attach it to the remaining switch position on the backside of the battery switch in the same manner as in Step 4. Assemble a cable in the same manner as in the previous step and attach one end of it to the accessory load position and the other end to the feeder terminal on the switch.
Remove the retaining nut from the small alternator field wire (labeled “F”) from the alternator with a wrench. Attach this wire to the alternator input position on the backside of the battery switch and tighten the retaining nut. Cut a length of #14 electrical wire long enough to reach the alternator from the switch. Attach wire terminals to each end of the wire, using the instructions that came with the terminals. Attach one end of this wire to the alternator with the original retaining nut then attach the other end to the alternator’s output position on the backside of the battery switch in the same fashion.
Hold the battery switch into position, lining up the mounting holes with the holes you drilled earlier. Tighten the four retaining screws (included with the switch) with a wrench.