Firecrystal

So how do I use a GuiHandler? So far I've made a class that implements IGuiHandler, registered it to my mod, and I'm not sure what a Gui ID is or if I have to register one or if there's a list of Gui IDs in use.

So I've so far been using Forge for my mod, because it allows for a whole lot of stuff to be done without modding base classes, and has quite a few impressive mods using it. The only problem I'm having is with the gui API, and the handlers in general. Is there some kind of documentation for how to use all these IWhateverHandler interfaces? As for the gui, vanilla Minecraft can open those easily using functions attached to the player, so why isn't there a function on EntityPlayer that I can pass a GuiScreen instance to and have it opened, instead of creating whole new classes just to implement the confusing interface? Is it something to do with SMP, or a security issue, or just some strange tenet of coding etiquette?