You lock the primary drive (Crank) gear and clutch basket gear with a penny, being copper it is soft so shouldn't damage the gears (If you look in the pic you should just be able to see the penny). Alternate the (4) steel and (3) fibre plates (steels on outside), with these clamped together they lock the clutch inner to the basket. The torque setting is 60nm or 44ft-lbs. I got these figures from a torque table some where on this site.
Torque table:
http://www.gasgasrider.org/html/manuals__etc.html