Take off clip holding outer casing of control wire, remove nipple from twist grip. Screw off ring holding throttle slide in carburetter, lift up throttle slide and unscrew inner wire from slide. Insert new wire by threading through, taking care to oil it well while doing so. Reassemble.
Shut twist grip off, see that throttle slide is closed. Slacken off clamping nut holding outer casing, slide outer casing up or down until all slack or lost motion is taken up, clamp tightly using a second spanner to hold bottom hexagon to avoid twisting holder off where it screws into carburetter.