Short instructions on installing games using disk images on Steam Deck.
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I'd probably recommend using AnyDesk to perform these operations remotely or having mouse and keyboard plugged in, as using touch screen makes the entire process extremely annoying.

Obviously you'll need an image of a disk (if you have a CD/DVD reader - you can create it from existing disk - not sure which formats "mount" supports, so I'd probably go with most popular iso). A problem may be finding a ripper that can create a CD image that is free (on Linux there are some ways to just do it as far as I know). Once you have an iso file, you need to copy it to a deck somehow - using Warpinator (on Linux/Steam Deck) and Winpinator (on Windows) and copying it over local network is one way to do it. Anydesk can probably also copy it, but it might end up sending it over the internet. Finally, the game executable will likely have to be cracked, to make sure it doesn't require a CD. It should be possible to set up Steam to mount an image on game launch, but I haven't experimented with that yet and it probably won't be enough to fool anything but the simplest copy-protection systems.

First of, a user password is needed (because for whatever reason the main user has none and sudo needs a password). You can set by opening Terminal and using "passwd" command.

After that you need to create a folder which will be used for mounting:
- "sudo mkdir /run/media/virtualdrive"
(first use of sudo will require typing in password for current user)
Then you need to mount your image in that folder, for example:
- "sudo mount -o loop /home/deck/Warpinator/NFS\ Most\ Wanted.iso /run/media/virtualdrive/"
Make sure to use Tab to autocomplete commands.

Once it is mounted you go to your Steam in Desktop mode -> Add a game -> Add a Non-Steam game.
Click browse, navigate to a virtual drive, so:
"/run/media/virtualdrive"
Under file type select "All files" and find a installer executable (usually install.exe or setup.exe - alternatively autorun can also be used).

After you add it, it will show up in Steam. Right click on it and click Properties. Under compatibility, tick "Force the use of a specific Steam Play compatibility tool" and select any new Proton version you have installed (you can find them under in your Steam Library under Tools).

Then you just click play - an installer should show up and you install the game.
After the game is installed, open folder browser (Dolphin on Steam Deck by default) and navigate to:
"/home/deck/.local/share/Steam/steamapps/compatdata/"
There you'll find a bunch of randomly (or pseudo-randomly?) generated catalogs - one of those contains your installed game with its Proton settings.
If the game requires a crack to run without a CD/DVD - apply one. Once you are done, edit the entry for the game in Steam (the same one you used for installation). Under shortcut click browse and navigate to:
"/home/deck/.local/share/Steam/steamapps/compatdata/<unique id>/pfx/<path to the game as if it was on Windows>"
Select the executable that is used for launching the game.

Click Play - if the game launches, close it and return to Gaming mode and launch the game from there instead to be able to more easily set up controller configuration etc. If the game doesn't launch, still going into Game mode and seeing it it helps, as proton sometimes seem to glitch out.