Use visual studio a Development enviroment.
The community version is free.
Design C# application with UI.
You have 2 option
Winform -> easy to use, but not most recent.
WPF -> steap learning curve but more versatile and future proof.
Tim Corey on youtube
Are you asking specifically about UI design in the Unity game engine or general UI design? There's a big difference. There are also differences between operating systems unless you use a cross platform toolkit.