It's a very interesting assignment, and it's not that hard once you understand what you have to do.
But anyone who follows a professional methodology will recommend that before you make something from scratch, you look for something that can do the job already.
I'm not familiar with any dungeon-crawler SDK, but then again I'm not particularly knowledgeable in this genre. Maybe in some dungone-crawler enthusiast forum, or modding an existing game with mod tools?
If you work on an editor and engine and make them good enough, they may even be desired by other people such as yourself that want to make a game with this style.
In any case, you've never programmed an editor before. It's a GUI app, like MS-Paint, Notepad, Word, Excel - that is, it's an application that has an interface built with the OS's GUI controls (menus, panels, buttons, checkboxes, spinners).
In your case, along with all your menus and buttons etc. you'd also want to have an OpenGL 'canvas' control that you use with SFML to render your editing area - that diagram and nodes discussed before, with thumbnails etc.
First you design your application with pen and paper, away from the keyboard. You write\sketch how you want your editor to look, what kind of menus and buttons it'll have and how it'll work for editing.
Once all the design is in place, you need to implement it in actual software. For this you can pair up QT with SFML - QT is a very good GUI library, comes with a lot of modules and has a visual editor for you to place controls and handle layout. There should be more information in the SFML community forums as well.