Go to
https://github.com/Zaela/p99-login-middlemand and click the green "<> Code" button, then click download zip.
The zip file will be downloaded to your computer, probably in your Downloads folder (unless you changed it). Go to that folder (the easiest way is to right click the download in the download list in chrome, then pick "open containing folder" or something similar), then right-click the zip file and extract it.
(You may want to move the extracted folder somewhere besides downloads, but it doesn't really matter.)
Open the terminal program (I don't have my steam deck in Linux mode ATM, but this should involve clicking the "Start menu" and finding Terminal, or something similar like "Console"). Inside that terminal, use the "cd" command to change your directory where you extracted the files. For instance, if you didn't move them out of downloads, the command would be something like "cd ~/Downloads/p99-login-middlemand" (and if you moved it somewhere else, use that directory instead).
Now, you're in the folder at the command line, so you're ready to actually "install" (really "build") the program. To do this, simply run the command "make" at the command line. If you get any errors, you probably just need to install some package; google the error and you'll find a "sudo apt-get install *someLinuxHeadersPackageOrSomething*" command to fix it. Run that command, then try "make" again.
Now your software is "installed" (built), so you can run (at the command line, from the same directory) the command "./bin/p99-login-middlemand" to start the program.
After that, all you have to do is edit your eqhost.txt file (in your EverQuest directory) as described on
https://github.com/Zaela/p99-login-m...readme-ov-file (change your's to look like that). Everquest should now work, without that annoying missing server issue.
Note that you will have to restart middlemand every time you restart your computer (or close that terminal). The steps will be simpler though: you just have to start the terminal, "cd whatever/directory", and then run "./bin/p99-login-middlemand".