How Do People Make Those Guis For Suggestions?

So most of the suggestions I see have images of a custom GUI that gives an example of what it would look like in-game. My question is, how do you make those? I'd really like to be able to add some to my suggestion

 

From what I can tell, its people taking the Wynn texture pack to a singleplayer world, putting down a chest, and rearranging items inside the chest so it looks like a GUI. For instance red stained clay has the icon of a red X in the Wynn pack, so a person would put that in the chest to symbolize a red X in their GUI.

 

Thanks, but how do they rename the items to make it look like a professionally made wynncraft gui?

 

Commands most likely. I haven't played vanilla minecraft in forever so I don't know how exactly to do it, but there are ways to change an item's name and the color of the text and whatnot.