1. Enable Developer Mode
2. On Developer options, enable OEM unlocking and USB debugging.
3. Connect via adb
4. Run adb reboot bootloader to get to fastboot
5. Run fastboot flashing unlock
6. Run fastboot reboot
7. Go to https://developers.google.com/android/images and download the image that corresponds exactly to your devide and Build number. In my case

https://dl.google.com/dl/android/aosp/akita-bp4a.260105.004.e1-factory-d9bc5fb8.zip

1. Extract the .zip and you’ll see another .zip inside. In my case was named image-*.zip, decompress that one too.
2. Get a file named init_boot.img from the second zip. We just care about that one, you can remove the rest.
3. Push that file to the phone via adb
4. Install magisk on the phone (the normal apk). Use github, care with malware.
5. On the magisk app, click “install” and then “Select and Patch a File”. Pick your init_boot.img
6. Click let’s go.
7. Now you should have a new file on the device named magisk_patched*.img
8. Pull it to your PC
9. Run adb reboot bootloader
10. Now flash the pulled image with fastboot flash init_boot magisk_patched*.img
11. fastboot reboot
12. Now on the magisk app you’ll see “Magisk. Instal”