A diagram probably isn't really needed. It's very simple, really. The ignition switch (the key-hole) gives power to the starter button. The starter button gives power to a solenoid. The solenoid gives power to the starter motor. When you press the starter button, it closes the connection in the switch, which then closes the circuit, and transmits the power from the ignition to the starter solenoid. When the starter solenoid gets that power, an electromagnet magnetizes, and closes an internal circuit in the solenoid. At one side of the circuit is the battery, at the other end is the starter motor. When the internal circuit is closed, the starter will run.
So, if the bike is constantly starting when you turn the key on, your switch is bad (stuck in a closed circuit). You can take the handlbar controls apart, and trace the two wires from the starter switch. Then, take a multimeter, set it in continuity mode, and see if those two wires are "open" (no continuity, AKA, no connection) or "closed" (continuity, a connection is being made between them, AKA the switch is stuck internally).
If the two wires are showing continuity, well, your switch is stuck. Odds are good you'll have to replace it, but maybe getting a tiny bit of oil into the switch and then tapping on it while you work it back and forth might get it to free-up inside.
I really feel this is the only feasible possibility.
Doug
edit: Or, you got water in the switch. That could be "closing" the circuit, too. If it still acts this way after trying to tap and work it, maybe stick a blow-dryer on it for a couple of minutes. That should evaporate any water inside.