you need a bottom bracket wrench 16 notch or a disc brake multi tool

on the hubs is a transport safety spacer

open the lock ring

take the lock ring, the washer and the paper spacer off

put the disc brake rotor on the hub

the outer side has the toothed base for the lock ring

rotor on the hub

tighten firmly

Locking rings with both internal and external serrations are available. These lock rings have teeth on both the inside and outside.