Assuming everything is connected at the twistgrip, slacken off all the adjustment in the cables first.
Locate the cable ends in their respective places at the carbs, and ensure the inners sit where they should be. You'r kind of working backwards if you see what I mean.
If you then keep the cable taught by pulling back on the outer, you should then be able to slot the adjusters into the brackets and adjust.
It can be a bit fiddly, but unless you've got hands like the Incredible Hulk you should be ok.
Hope this helps.