• What you need:

  • A Samsung Galaxy S20 FE

  • For TWRP:

    • Odin from samfw.com
    • A patched vbmeta image I found this one from this XDA post
    • And of course, TWRP

  • For Lineage OS:

    • You can follow this guide on the official LineageOS website

Firstly,

By doing this, your warranty is now void and Knox is tripped.
Tripping Knox will cause you to lose out on many Samsung features and it is irreversible,
so make sure you understand what this will do before you attempt this.

I am not responsible for any bricked devices, broken SD cards; this is your fault, not mine.
You chose to do this, not me.
This is just how I did it.
It may not still work when you attempt it, please be carefull and double check all info.

I recommend reading up on what tripping Knox will do, and the risks of bricking your device before following this.

Also, please read all of this post before attempting this.

You will need to now unlock your bootloader,
The way I did this was by setting my device clock back at least 7 days,
powering off my phone,
plugging it into your laptop/PC and pressing the up and down volume keys at the same time,
finally, pressing and holding the volume up key.

If you followed all of those steps exactly, the bootloader is now unlocked and your phone has factory reset itself.

Next, open Odin on your PC and select the twrp tar in the AP slot. To do this, just press the AP button and select the TWRP file.
Also, put the patched vbmeta into the userdata slot the same way.
Now you can press the start button in Odin and if all has gone to plan, it will reboot your phone.
Now you enter recovery on your phone by powering it off, pressing and holding the power button and the volume up button.

If you followed this guide exactly, then congrats, your phone has successfully booted into TWRP. Yippee!

Once you’re in TWRP, you will need to format your data, metadata, keydata, and keyrefuge partitions to use your phone again. To do this, just go to Wipe > Advanced Wipe and select those partitions.

TWRP is now your custom recovery!

To apply a custom ROM, you can simply wipe > format data > yes (now your phone has been fully reset).
Now you use your PC to drag and drop the image file of the ROM you want to apply into your phone’s main storage.
Go to Install > select your image file and flash.
If all went correctly, the next time you boot your phone, it will boot into your custom ROM!

Important,

This is just how I did it, and I followed a mix of XDA posts and my friend’s advice, but the most reliable post I found was this one. So please cross-check any information you find in this post with other sources as this may not be the most updated guide. Remember to only download from trusted sources and follow guides you know are 100% factual.