The ignition switch has the following connections ;-
G - Ground
M - Magneto (Connected to ground when switch OFF, open otherwise)
B - Battery
L - Load (Connected to 'B' when switch ON, open otherwise)
S - Start (Connected to 'B' when switch at 'START', open otherwise)
If you are starting from scratch, start by connecting the battery -ve to chassis ground with heavy cable. Connect heavy cable from battery +ve to one large connection on solenoid. Connect other large solenoid connection to the starter motor.
Assuming you're not worried about the safety interlocks and any dash lights, proceed as follows ;-
Run a wire from battery +ve to 'B' on ignition switch.
Run a wire from the engine magneto (probably a single black wire emerging from under the cowling) to 'M' on ignition switch.
Run a wire the alternator under the flywheel (or the red wire on regulator depending on model) to 'L' on ignition switch
Run another wire from 'L' on switch to any ignition controlled electrics (light switch, electric blade clutch switch etc).
Run a wire from the small terminal on solenoid to 'S' on ignition switch.
Run a wire 'G' on ignition switch to chassis ground.
Hope this helps!
Phil Saunders, April 2009