You'll feel when the nut makes contact, sandwiching the washer assembly. IIRC, a 10mm socket fits over the top of the shock shaft. I used a ratchet on that and an open-end box wrench on the nut and squeezed to an approximated 18-20ft-lb. RealOEM doesn't have torque specs so I assumed this based on previous M3 specs. You can always torque the nut down after the assemblies are installed on the car. This may even be easier since the shaft shouldn't spin with the car on the ground under the spring's tension.
2013 Mineral White e92 M3 DCT ZCP
2012 Bluefire S1kRR Race DTC ABS QS HG
2008 Sparkling Graphite e90 M3 6spd XXX