[Google Pixel Guides, News, & Discussion] 32BitWhore:
Yeah it's a mess finding a current guide for installing Magisk, especially after the May update.

Here's what I did. It looks a lot more complicated than it actually is.

This is assuming you already have platform-tools (adb/fastboot) installed and working on your computer.

1) Download the July full OTA image for your carrier (O for Verizon, M for T-Mobile/Fi, B for Deutsche Telekom, D for all others).

2) Download twrp-3.0.2-0-RC1-fastboot-sailfish.img to your computer.

3) Unzip the OTA image to a folder of your choosing (or to your platform-tools folder for ease of use if you don't have platform-tools in your $PATH)

4) Open flash-all.bat (PC) or flash-all.sh (Mac/Linux) in a text editor. Remove the "-w" from the last line of the file if you want to save your data/apps/settings. If you want a clean flash, skip this step. If you don't have platform-tools in your $PATH on Mac or Linux, you'll have to add a "./" before each line that starts with "fastboot" in flash-all.sh. Save and exit the text editor.

5) Download the following to your phone: twrp-pixel-installer-sailfish-3.1.0-0RC2.zip (TWRP RC2), Magisk-v12.0-Pixel.zip (Magisk V12), VerifiedBootSigner-v8.zip (boot image signer), and ElementalX-P-1.14.zip (ElementalX Kernel)

6) Connect your phone to your computer at this point if you haven't already, make sure you have USB Debugging enabled in developer options, then run adb reboot bootloader (or adb reboot-bootloader on Mac/Linux)

7) Open a command prompt/terminal, navigate to the folder where you extracted the OTA image and run flash-all.bat (PC) or ./flash-all.sh (Mac/Linux)

8) Let your phone reboot and finish the update (first boot can take a bit longer than usual - mine took 2 or 3 minutes IIRC)

9) Once the update is complete, run adb reboot bootloader (or adb reboot-bootloader) again.

10) Run fastboot boot X:\Path\to\twrp-3.0.2-0-RC1-fastboot-sailfish.img. This will boot phone into TWRP RC1.

Note) If you're using a lockscreen and phone encryption, make *sure* TWRP asks for your password. If it doesn't, repeat steps 9 and 10 until it does. It can take a few tries sometimes because TWRP is buggy. Flashing anything without decrypting your phone can and will corrupt your data resulting in the need for a complete wipe.

11) Select 'Install' and flash twrp-pixel-installer-sailfish-3.1.0-0RC2.zip.

12) Go back to main TWRP menu and select 'Install' again and flash VerifiedBootSigner-v8.zip

13) Go back to main TWRP menu and select Reboot > Recovery.

14) You should now be loaded in TWRP RC2. (Again, make sure it asks for your password - if not, select "Read Only" from the prompt that pops up and repeat step 13 until it does). Go to 'Install' and flash the Magisk-v12.0-Pixel.zip file.

15) Flash the ElementalX-P-1.14.zip

16) Reflash the twrp-pixel-installer-sailfish-3.1.0-0RC2.zip or you won't have a recovery installed

17) Reflash VerifiedBootSigner-v8.zip

18) Reboot system

19) Download the latest stable version of Magisk Manager from APKMirror to your computer

20) Install the apk by running adb install -r MAGISKMANAGER.apk. the -r flag will reinstall it in case it was already installed.

21) Open Magisk Manager, go to settings and select the "Core Only Mode" option or it will not work properly.

22) Also in Magisk Manager settings, enable 'Magisk Hide' and then reboot.

Should be good to go after that. Hope that helps.