If you are still on stable:
1. Boot twrp
2. Flash magisk canary.zip here
3. Reboot to system and install Open beta with updater Don't reboot!
4. Go to magisk and install to other slot (after ota)
5. Now reboot
That's what I did yesterday and it worked without issues.
I tried other method with patching the boot.img but got bootloop somehow. Not sure what was the issue.