The easiest point to jump on is KDE-Plasma, look it up, it is a complete DE that is very customisable and it is all doable through a GUI - themes, widgets etc its all baked in. You can do a lot of customisation in plasma fairly simply, that might even be enough for you, you can really do a lot in plasma and you don’t need to know much besides getting it installed on your system.

From there, if you want to touch something a bit more complicated you could install some window manager to use alongside it and just fiddle with that, look at the configs, understand how that works, try to build your own thing from the ground up, or just copy someones dotfiles here.

E: To clarify, if you use a display manager like SDDM it will let you select the DE or WM on login.

There is no universal guide. It depends on whether you are using DE or WM and which one. Then, if it's WM, you need to find other things, such as a terminal emulator, application launcher, status bar, etc. For example, I'm currently configuring Hyprland with kitty, rofi, eww and a few other things. All of them have their own wikis and docs. I would advise you to start with something simpler or use someone's rice, some of them come with installation scripts.

If you're using kde as your desktop environment I'd look at themes using kvantum Manager. You can also mix and match other themes through the default app store or whatever they call it

The GitHub pages usually have information about how to install stuff. There are also YouTube videos and articles online you can easily find with a quick google search about this. Use your brain