Without the specific shop manual in front of me.
No1 Cylinder on TDC
cam pulleys on TDC possibly need to have boths cam shaft pulley slightly before TDC to get belt on May also need to restrain cam shaft pulleys with spanners/tools in the correct position to put belt on
put belt on so that all the slack is on the belt tensioner side, let belt tensioner self tension the belt (ie no added pressure) Tighten the belt tensioner bolt.
now do a couple of revolutions of the engine with the plugs out checking cam shaf/crankshaft alignment. After sonfirming correct timing proceed to next step.
With plugs in rotate engine 12 teeth on the cam pulley in the normal direction Then go back 6 teeth, now undo tensioner securing bolt and let the tensioner self tension the belt (NO ADDED TENSION) then retension the tensioner securing bolt.
Re check cam timing.