Don't do that! All of the current required by the items fed by the fuse panel would need to come up the engine to helm harness and then through the ignition switch. Neither are designed to handle that current. Only the instruments are intended to be powered by the ignition switch. Yes -- some small load devices could be handled, but not an entire fuse panel worth of electrics. You run a separate #8 or #10 pair of wires from the battery to the fuse panel at the helm. If you want a MASTER switch, use one of the switches on the helm, or add one. That switch switches +12 volts from the battery to the fuse panel. You also need a 20A fuse or breaker in the red wire within one foot of the battery. But -- if you insist on doing the ignition switch route, you run a #8 or #10 wire from the "I" terminal on the switch to the +12V buss on the fuse panel. The ground buss on the fuse panel (if it has a ground bus) connects to the large ground wire in the engine harness (if it has one). Otherwise there needs to be a separate #8 or #10 ground wire running back to the battery. A ground bus can be added if the fuse panel doesn't have one.
