Step one is to get yourself an ignition switch. Big tip here is not to buy it from a motor accessory shop, buy it from a boat yard. You'll spend more money, but you ought to get a switch that doesn't object to getting wet, which can be a big bonus on a cold, dark, wet night. You're ideally looking for a switch that has four terminals on back marked as per the diagram. This will usually mean the switch has three positions to it, OFF, ON and START. Switches with more positions or terminals tend to have odd ball positions for park and accessories without ignition. If you mount a keyswitch low on the left hand side of the bike (somewhere under your ass is good) then you'll keep the wire runs short and be able to start the bike with your left hand whilst keeping your right hand on the throttle ,unless of course it's kick only in which case you'll only need a simple ON/OFF key switch which may have only two terminals. In that case just hook everything that I've shown on the ACC terminal and the ignition to the same one terminal

A hint here about fitting key switches. It is well worth taking the time to file out a proper oval shaped hole for the key swich as it's really annoying when it turns in a round hole and stops you from using the "START" position, or shorts all the wires on the back of it out and blows all your fuses.

Big fat wire, and a 30 amp in line fuse (near the battery) connected to the "BATT" terminal on the back of the switch starts your wiring loom off for you. At this stage it's worth adding a cable for the earth but not actually connecting it yet, as this avoids lots of sparks and other such exciting moments. The earth cable needs to be the same size as the feed to the starter solenoid if you have an electric start, so a cable and not a wire....