How can I create a GUI?

So, I am in the planning stage of the plugin and I am already stuck.
What I am aiming to do is create a GUI that actually has a place where the user can input data, like a text box input. The issue here is that I do not know where to even start. My hunch is that I will need to look into something similar to the bossbars and TAB menu changes that are made by the CMI plugin. So, how exactly does CMI make these GUI changes? Where do I start?

 

As far as I know, you can't actually make your own new type of GUI with Bukkit, though you can utilize those already created in the game. If I needed a text input from a user, I'd use the anvil GUI, as you can type in an input. Mineplex uses this to let players search for others to send friend requests to.

 

Using the sign GUI is also an option: https://bukkit.org/threads/sign-gui-use-the-sign-interface-to-get-user-input.177030/

 

This util allows you to get input from a user using the GUI of an anvil. I've already used it myself and I have never had any issues with it.
@KingOfTheEast01 I guess that's what you meant.

 

Yeah, that would work, though the GitHub page supplied in that post isn't working for me. He could create his own anvil inventory if he wished to though, and just play around with it to get the best results.

 

Oh, thanks, I did not notice that. Fortunately, a user posted an updated version of the class. I'm not sure if it's the same as the original but it's certainly worth giving it a try.