*Normally*, you need to split the RH switch block that the throttle rotates in. Then , thread the cable and nipple end through the hole on the lower part of the switchblock. Locate the nipple in the appropriate 'hole' in the throttle housing, and ensure the cable sits in the 'slot' properly. Now is a good time to spray a bit of WD40 on the inside of the throttle housing, and also down the cable if you have one of those trick cable-spray thingy's.
The throttle housing should rotate freely on the handlebar. It now needs to be 'located' properly on the lower half of the switch block, then assemble the switch block . That *should* be it........if you have 2 cables, then same procedure above applies for both cables.........don't forget to adjust the freeplay, usually this is done with an adjuster located immediately under the switchblock. You only need enough freeplay so that your throttle has a small amount of movement, 1 or 2 deg is enough.
Hope this helps, pm me for details of you need more.......