I haven't seen any mouse release methods in lwjgl. How would I go about this, yet keeping things simple?
Just go into it knowing that it's not going to be simple. I have great appreciation for the developers who have written major UI libraries.
Abstracting raw mouse/key events into an event dispatch system is tricky especially when you consider drag/drop, focus, window z-stack, and modal.
_If_ you want want to write things from scratch count on spending at least a month full time to get something useable and definitely not complete. This is considering you are working off the shoulders of giants e.g. looking at how awt, TWL, nifty, etc. are implemented.
You will have to start with a UIComponent that stores a location + dimension, can paint itself, have children, and can handle events.
I've probably spent between 30-45 days on mine. I think it's worth it in the long run - good learning experience and I have a framework that avoids spaghetti code. If you don't want to spend that time e.g. would rather write your game then your only option is to learn a library.