Install Magisk / root your phone

You need to extract your boot.img first (see other page)

install magisk app (make tripple sure to pick the right one ... bad people out there ...)
copy boot.img on your device
open app, hit install
selekt your boot.img
you wont finde the patched img on your PC due to RW permissions, i suggest you to share the file via DataExplorer via an messenger/encrypted e-mail)
reboot to bootloader
flash patched boot img

fastboot flash boot PATCHED_BOOT.IMG