It's not that hard for a 2012 machine, I explained what steps are needed in a post a few pages back, search my history for it, I will link to it shortly in this post, will edit this post.
Link
https://forums.macrumors.com/thread....2242172/page-241?post=29221192#post-29221192
1. Have a USB stick handy, or an external disk, or a dedicated partition.
2. Download the full installer.
3. Download Micropatcher from
here.
4. Open Terminal.
5. sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/Install\ macOS\ Big\ Sur\ Beta --volume /Volumes/***
*** is the disk label you install the Installer onto, so the USB stick, I dragged and dropped my USB stick onto the terminal, the path will be automatically added.
6. Input your password.
7. It will erase your disk so be aware not to chose the wrong one.
8. Input "Y" if asked to erase.
9. Wait until it's finished, could take a while dependable of what kind of media you use.
10. Unzip the downloaded micropatcher, drag the micropatcher.sh file onto the Terminal window, then enter.
11. In Terminal type sudo and then a space, next drag the install-setvars.sh onto the Terminal window, hit enter.
12. Restart after it is finished.
13. Hold Option key right after restart, keep holding it until you see the bootpicker.
14. Chose the EFI disk, it will instantly shut down.
15. Start up again and again hold the option key for the bootpicker.
16. Chose the USB installer, wait for the installer to open.
17. Chose the disk you want Big Sur installed onto.
18. Wait for probably an hour or so (Have a coffee or a beer in the meantime 🍺 ) if you install from a USB stick, probably half of that if the installer resides on a SSD medium.
19. Restarts multiple times.
20. Almost done.
21. The next step is for WIFI, Wifi does not work OOTB on a 2012 Mini or 2012 MBP....
21a. Restart for the last time after it booted into Big Sur, you may restart before login or after login, makes no difference.
21b. Hold the Option key again to get into the USB installer, there are other ways but this one is the easiest, chose the USB installer, next open terminal from the menu.
21c. Input "/Volume/Image\ Volumes/patch-kexts.sh" and hit enter, when finished enter "reboot", both without the quotes.
Voila...done...a working Big Sur OS.🍺🍺
Edit: Do not forget to thank Barry K. Nathan