The best way to do it is to hook the "ignition start" wire to the "start" terminal on your ignition switch (usually the center terminal) and the "run" wire to the "ign" terminal. If for some reason you want to tap into wires instead of hooking directly to the ignition switch, connect the "ignition start" wire to the signal wire that goes to the starter solenoid, and the "run" wire to any wire that has power with the key on.
BTW, If you accidentally hook the "ignition start" wire to a key on power wire instead of the starter solenoid wire, your engine won't rev above about 2000 rpm. Don't ask me how I know this
Pete