I'm not responsible for any issue you may incounter while doing the steps below.
Do it on your own risk.
SO! mauronofrio updated his twrp so there is no need in key master anymore.
Before you're doing anything, delete all your magisk modules and substratum layers.
The guide was already provided by mauronofrio, but i dunno why people still struggling on getting everything working fine. I'm explaining here my own steps on flashing everything.
1. Flash full OOS 10.0 zip via local update . (DO NOT REBOOT)
2. Flash fajita twrp 3.3.1-24.zip by @mauronofrio via Magisk Manager (DO NOT REBOOT)
3. Flash magisk to 2nd slot via MAgisk Manager (Magisk Manager → Tap on Install → Then select Install to Inactive Slot)
4. REboot and wait :3