This is the "standard" modern method that works without a custom recovery. It uses ADB to move files and Fastboot to flash them.
If you have TWRP already:
Connect your phone to your PC and run: adb sideload Magisk-vXX.X.zip install magisk with adb
Once your device is in Fastboot mode, commands switch from adb to fastboot . This is the "standard" modern method that works
Wait for the screen to change to the Fastboot/Bootloader menu. install magisk with adb