What are the necessary steps for building a GUI system?

All though the steps a relatively similar do you mean for in-game UI?

An in game gui with buttons, scroll bars etc.

In general prefer to use library if it's a viable option, if you really need to do it yourself for whatever reason there are some pretty good design principles to follow involving OO design patterns and combintation of them. The composite & decorator pattern would be a good place to start with since that is the structural basis of a typical widget toolkit (the widgets/GUI elements themselfs). A typical pattern combination for a typical widget toolkit is the composite, strategy, and observer pattern or more widely known as the model-view-controller (MVC) pattern. However more patterns than this can easily be needed so i recommend briefly getting familiar with all of them so you know what, why, how, and when something is applicable.