If you want to make one yourself, the skills you need is being able to paste images to the screen (most of the time with some text on them) and mouse input. I made a GUI that I use in my games and, let me tell you, it wasn't easy. A GUI ranges from the simple "clicking on buttons" to text editing (sounds easy, but it's not). I went all out with mine: buttons, radios, lists, dropdowns, free text boxes, horizontal sliders, and a file system. It’s likely you don’t want all that. Is it a 2D or 3D game? If you have a 3D game, you will want to be sure you know how to manipulate objects in 3D with the mouse. At any rate, you will likely need buttons and lists for even the simplest level editor. Most GUIs have a grouping system for the selectable objects in a particular menu—I call them “menu groups”. Each group has its own buttons/lists/whatever associated with it so that when that group is being shown then will be rendered and checked for state changes automatically. There is really no good tutorial that I have found, so I was forced to make my GUI by imagining what I wanted each thing to do and then figuring out how to make it happen. It’s not much different than any type of programming; you just need to imagine it and convert your imagination into code…. I hope this helps you get what you are looking for.