ok, it looks like the manual has a graphical install guide. I recommend setting up home-manager for stuff you want set up for your user. programs that need configuration like steam, firefox, etc. are enabled as options in your /etc/nixos/configuration.nix and home-manager configuration files. so you generally just need a programs.steam.enabled = true; in the appropriate file. you can see the available options for nixos here and home-managers options are in the appendix of the manual I linked earlier. simple programs that are just binaries go into either the environment.systemPackages list or into home.packages.

you generally only want stuff that should be available globally in the system config and most everything else should be installed locally for your user.

home-manager will even install some firefox addons for you if you want them and it will even let you set up userChrome.css and the like. basically, with nixos, instead of configuring programs/services by mucking with config files all over your system, you just set the appropriate options in the nix config files instead.

if you get confused at any point in the process, ping me and I’m happy to help. I can also link some tutorials on youtube if that’s helpful.