Unscrew the top of the carb and the throttle spring will come out along with the carby piston/throttle slide, the cable goes through a slit in the throttle slide.
The other end of the cable goes into the throttle housing, unscrew the screws on the throttle so it comes apart and connect the cable to the plastic part inside. Its a good idea to put some grease in there aswell to avoid the cable snapping after a while.
Thats about it without going into to detail.