Eh, I'm not an expert nor have I taken any classes about this whatsoever, but here's what I do to make GUI.
The first thing I would do is identify the theme of the GUI. It can be bloody... or just very pure, etc.
Then, I will think back: what is the main focus of the game? Is the game about family? Then maybe I'll add a family picture for the main menu background. Is the game about a particular person or people? Then maybe I'll add a CG of them for the MM bg. As for the other backgrounds (Prefs, Save/Load etc) I would usually put a logo there, but some other people have their ways. You can have chibis, decorations, or anything that really symbolizes your game. After you think through of what your backgrounds are gonna be about, then buttons/bars or any other additional stuff will just need to have the same pattern, color that will fit into the game.
Then, you can start making GUI! Usually, I start off with a base color, then some shading and random filtering. Shading is just using a brush with hardness 0%, and filtering is well, just the filters you see in the filtering dropdown box... Here's an example of one bg I did by that:
(Ik it turned out REALLY shiny... You can use it if you want for whatever needed... it was made for CC anyway lol)
I don't know how others make their background... but I always make my background like that >.>
Anyway, conclusion of my thoughts as a player: GUI does not need to be fanciful, complicated, walala with all the effects coming in... Most importantly the GUI must fit in your story.
Hope this helps (I hope I'm answering your question ><)
