How do I free up disc space to install Monterey?

I had around 30 GB free and Monterey update said I needed 12 GB more. I'm not playing a "dangerous game," it's that Apple's OS upgrade files are monstrously large, and they provide tiny hard disks on so many of their entry-level computers which is why this ends up being a problem for lots of people who have older hardware.

Delete stuff.

I had to free up 12GB on my MacBook Air to install Monterey. I started by removing big apps like Photoshop and Final Cut Pro that I could re-install after the update. You can also get an app called Disk Doctor that will show you a list of the largest files on your hard disk. You can either delete some of these larger files or move them to an external drive or USB stick, then move them back after the update. Just make a note of where the files were located before moving them.

Doing that, I was able to free up 14GB, install the update for Monterey, then re-install the apps I removed and move files back to my MacBook hard disk.

you can purchase an external HDD or SSD of sufficient capacity and move your larger files (videos, pictures, documents, etc...) to it. once they are on the EHD, you can simply erase the same files from your internal drive. and, if it's just going to be used for storage, you can use an HDD and save a few bucks over the cost of an SSD. FWIW, i personally keep very little on my internal drive.

You are playing a dangerous game if you allow your hard drive to almost fill up. Mac OS needs an absolute minimum of 10 - 15 GB of empty space at all times to function properly. If you fill it up completely, your computer will stop functioning. Your “storage” space also houses and runs the OS and all needed system files and needs room to write.

This isn’t just about large upgrades. It is about providing enough free space for the installed OS to function properly. I never allow my available empty space to go below 50 - 100 GB - the OS will thank you. Yes, I know the entry level models have tiny hard drives. I never buy anything below 512 GB. So, if you choose something smaller, you need to be more conservative and only have the basics on the internal - use an external for all the other stuff. Don’t blame the computer manufacturer. And I do have older hardware - it has a 1 TB size drive.

And yes, if you fill it up completely, it will stop functioning and it will be near impossible to access files to delete them, so maintain current backups.

You delete 'stuff' that you no longer need and/or move data to external storage to free up space on the boot drive.

Have a read of this Apple Support article: Free up storage space on your Mac - Apple Support