This is not so easy on a '78 with the three fuses. The wiring is pretty complicated and the ignition switch does more than just turn the ignition power on.
Get a real Honda service manual or owner's booklet wiring diagram and study it, don't waste time with any other wiring diagram. When you figure out what the stock system does you should be able to figure out how to modify it for a new simple ignition switch.
The relays are to take lamp and/or ignition current out of the ignition switch and a lot of harness wire so that voltage drop is minimized. Basically you wire the relay coil with the wires that normally power the load and provide a new fused power wire from the battery to the relay, connecting it so the relay switch powers the load from your new power feed when energized. DO NOT skip these fuses! They should be as close to the battery as possible, minimizing the amount of unfused wire connected directly to battery +12V: a short to ground will draw a huge amperage from the battery and quickly heat this wire to incandescence and it will start a fire on any combustible material it touches.
In the Honda harness you will see that the unfused power wire to the main fuse is as short as possible and also has an extra protective sleeve over it.
