[Samsung Galaxy S10 Guides, News, & Discussion] Rober7o:
Here I share the steps that worked for me (G973F) to install a custom ROM/MOD.

01. Download ASF3 and flash it through Odin (or the last one compatible with the ROM or MOD you wish to use).

02. Install Magisk Manager from here.

03. Extract AP and copy it to SDCard.

04. Open Magisk Manager, press Install and patch AP. It will generate a magisk_patched.rar on \internal storage\download.

05. Copy magisk_patched.rar to your computer.

06. Reboot your phone to download mode.

07. Flash magisk_patched.tar as AP in ODIN, together with the BL, CP and HOME_CSC files. Important: Uncheck “Auto Reboot” in Options!

08. Unplug USB cable, press Power + Volume Down to exit download mode. As soon as the screen turns off,

immediately

press Power + Bixby + Volume Up to boot to recovery partition and continue pressing the volume up button until you see the stock recovery screen.

09. In the stock recovery menu select Wipe data/factory reset to wipe the data of the device.

10. Select Reboot system now, and

immediately

press Power + Bixby + Volume Up. After seeing the bootloader warning screen, release all buttons.

11. The device will automatically reboot for the first time it boots, this is completely normal and done by design.

12. After the device is booted up, do the usual initial setup. The following steps will need internet connection.

13. You shall see Magisk Manager in your app drawer; if not, manually install it again and continue to the next step. The app would be a stub and it shall automatically upgrade to the full Magisk Manager when you open it.

14. Magisk Manager will ask to do additional setups. Let it do its job and the app will automatically reboot your device.

15. Download TWRP from here and copy twrp_ianmacd.img to your SDCard.

16. Open Magisk Manager, press Install and patch twrp_ianmacd.img from the step before. It will generate a magisk_patched.img on \internal storage\download.

17. Copy magisk_patched.img to your computer and rename it to recovery.img

18. In your computer open magisk_patched.rar with 7-zip, remove recovery.img and then add recovery.img from previus step to magisk_patched.rar.

19. Reboot your phone to download mode.

20. Flash magisk_patched.tar as AP in ODIN, the rest of the fields remains blank. Important: Uncheck “Auto Reboot” in Options!

21. Unplug USB cable, press Power + Volume Down to exit download mode. As soon as the screen turns off,

immediately

press Power + Bixby + Volume Up to boot to recovery partition and continue pressing the volume up button until you see the stock recovery screen.

22. Now you are in TWRP Recovery, format Data and then reboot to recovery.

23. After the device is booted up, do the usual initial setup. The following steps will need internet connection.

24. You shall see Magisk Manager in your app drawer; if not, manually install it again and continue to the next step. The app would be a stub and it shall automatically upgrade to the full Magisk Manager when you open it.

25. Magisk Manager will ask to do additional setups. Let it do its job and the app will automatically reboot your device.

26. Download the ROM / MOD of your choice to your phone SDCard.

27. Reboot your phone, as soon as the screen turns off,

immediately

press Power + Bixby + Volume Up to boot to recovery partition and continue pressing the volume up button until you see the stock recovery screen.

28. Once you are in TWRP install the ROM / MOD of your choice but don't debloat (if the ROM / MOD has this option).

29. Once install finishes don't reboot, go to TWRP and reboot to Recovery.

30. After the device is booted up, do the usual initial setup. The following steps will need internet connection.

31. After that you can boot to TWRP and install the ROM /MOD again debloating what you want, again if the ROM / MOD has the option.

As an advice, I don't recommend to debloat Chrome.

I hope that this can help someone :)