You can get a special tool or you can make one by wrapping tape around the clutch pushrod and poking it through the friction plate until it seats,then moving the friction plate so its as square as possible.
Tighten the 6 retaining bolts enough so that the friction plate is gripped but
still able to move under slight pressure.
This ensures that the gearbox input shaft can locate properly into the friction plate and align it accurately.Lube the splines with Moly paste.
Fit the gearbox and turn the motor so you can see through where the starter motor goes that the friction plate is turning and centred.
Tighten the 6 bolts in stages up to their final torque as you turn the motor (i used the alternator drive pulley on the end of the crank with the plugs removed).Fit the clutch pushrod/slave cylinder and check it operates properly.
Thats how i did mine 6000 miles ago and it hasn't gone wrong so far.
