Just did this. Didn't need it for the 1.25" thick Genmar riser, but did need it when I replaced that with a set of Heli Bars.
Take apart the lever for fast idle and pull out the cable. Back off the clamp on the right side of the steering head so the idle cable can be pulled free.
Remove the two bolts at the front of the gas tank and lift the tank up.
Feed the idle cable between the two bolts at the fron of the tank. Slip the cable between the fork tubes, but on the left side of the steering head.
Bolt the tank back down and tighten the cable clamp on the right side of the head.
Route the cable back into the fast idle lever and bolt it together. You should now have a LOT of excess cable length and it will kind of loop ahead forward of your bars.