Look at these two manuals, one for the 7.5 RTR (your buggy) and the other is for a 777. Both show you how to install a clutch. Pick the one appropriate to your engine. Hint: it will be for a non-SG shaft engine.
Engine with a pilot shaft (SG)
http://www.kyoshoamerica.com/manuals/31777B.pdf
Stock GX21 with a clutch nut (non-SG)
http://www.kyoshoamerica.com/manuals/31275EU.pdf