Taken from the RealmCrafter: Standard documentation
Change the Character Selection Screen Background Edit
Change the set.b3d file with one of your own, and be sure to include textures in the same directory.
Use E_set.b3d if you wish to use an encrypted file. This file is encrypted when exported to RC from Architect.
Add Music to the Game in character creation/selection/title page Edit
Place a file named "Menu.mp3" (or .ogg, .mid, .mod) into the \Data\Music folder for background music.
Add movie (optional) as game intro to the game Edit
Place a file named "Menu.avi" (or .mpg, .mpeg) into the \Data\Game Data\ folder
Edit the file named "Menu Background.bmp" in the \Data\Textures folder.
You can add a logo for your game to the character selection screen by editing the file named "Menu Logo.bmp" in the \Data\Textures folder.
GUI Sounds Edit
You can set up GUI sounds in the client by changing the sounds for beep and click in the /data/UI/ folder
Tutorial from community member Arindale Edit
Your Project\Data\UI holds all the graphics for the windows and buttons in game. They are individual parts for bottom top left right and the corners with 3 different files for each part representing normal highlight and selected or pressed forms of the button. They each have a letter appended to the end of the name to show which button state it is. No letter means normal not highlighted and not pressed/selected. D means Selected/Pressed and P means Highlighted. Kind of backwards but if you experiment by changing them and testing the results in game you can see what I mean. That's how I eventually got it all done.
ButtonBottom.bmp - Bottom boarder of all buttons in game. Normal state
ButtonBottomD.bmp - The Pressed/Selected state ButtonBottomP.bmp - The Highlight state.
What this means is the bottom of ALL buttons use ButtonBottom.bmp and when you move the mouse over the button the P version is drawn instead. That is the highlight. When you click it the D version is drawn instead. If its a click button its drawn as long as the button is held down and then goes back to highlight. If its a select button like when you pick your character at the char select screen it will stay the D version until something else is picked (Another char for instance)
This folder also contains the close checkbox and minimize buttons for in game windows as well. It also has the Mouse cursor graphic and the window title graphics for focused and unfocused windows. ProgBarBackground and ProgBarForeground are the graphics for ALL the progress bars in the game. That is any attribute bars the quit bar the sound volume bar all of them. Background is the actual bar. You can resize this graphic and make it as fancy as you like. Mines 256x32 I believe is what I made it and has an actual bar with room inside for the Foreground. The Foreground is the progress the bar makes across the background. It only needs to be a few pixels wide with no patterns because its stretched the full length of the bar as the bar fills. It should be grayscale as well because its colored per bar in game.
The fonts folder in this folder is for changing the font. You can make it whatever font you want as long as you make sure you don't go too far outside the size of the original font. Since you cant edit the .dat for the font you have to make sure they match up pretty well. I redid that font you see in my screens like 5 times completely from scratch until I got it looking like it does now.
Colours.txt is a file in this folder that has values for changing certain in game window attributes like boarder width and color for lots of things. I haven't touched this file yet to see how it works exactly its kind of hard to read cause its in a compressed format but I am pretty sure you can use it to tweak certain settings for the in game windows. I mainly want to change the thick white boarder that I mentioned is the only graphics you cant find in any of the folders.
Your Project\Data\GUE the folder for the actual editors interface and doesn't need to me changed.
Your Project\Data\Gooey Gooey is the way GUI is suppose to be said out loud as if you were talking to someone. I personally think its stupid and I call it G U I maybe its just me. Anyway this folder is basically a backup of the real GUI folder and I believe this was the original name of it and is now obsolete. At any rate changing this graphics do nothing to the game.
Your Project\Data\Textures\Menu This folder contains ALL the buttons for the menus OUTSIDE the main game. That's the start screen. Login server select char select and creation options screens so on and so forth. The buttons IN GAME are made of the files in the GUI folder and the words are the game font. This folder contains 3 states for each button just like the GUI folder the exception here is that the selected/pressed state is never used that I've seen. Just Highlight and Normal.
Example: BCancelCharU.PNG - The Cancel button seen in Character Select Normal or UP state BCancelCharH.PNG - The Highlight state
BCancelCharD.PNG - The Pressed/Selected state. UNUSED AT THIS POINT as far as I can tell.
The D state unused applies to all buttons in this folder. I've changed every single U and H state and never once seen the default unchanged D state come up in my menus. That's why I think its completely unused at this time. I made my buttons out of the same parts I used with the in game windows I just copy/pasted the parts into the button windows and went from there. There's only 3 basic button sizes that I can recall and 1 stray button that doesn't fit any of the other sizes. The sizes are the long buttons as seen on the start screen the normal buttons as seen everywhere else like char select and creation and the big square buttons that have the rotational controls in them. The odd man out button is the Done button for the options menu its doesn't fit the shape of any other button so I had to make it from scratch from the button pieces the rest I used blank template buttons I made for fast copy/paste.
As you can see in this folder there's single graphics for the windows outside the main game. Menu.PNG is the background of the Start Screen only. Its stretched to cover it not tiled.
Your Project\Data\Textures\GUI This folder contains the action bar and big xp bar as well as the background for the buttons on the action bar. It has JUST the backgrounds of the in game windows like Inventory Trade Group etc. Its just the background of those windows no boarder no nothing. It also has the graphics for the slots in the inventory screen. EmptySlot.bmp is just what it says it is. The slots on the action bar. Backpack.bmp is the general inventory slot background. That's all the 32 slots under the equipment slots. Its black by default which means its completely transparent so you see the boarder made by the button files and nothing else. This folder also has the up and down arrows for changing action button sets and the small icons for the window selection area of the hot bar. The highlight and pressed states for all these buttons are controlled by color changing them with colors controlled by the Colours.txt mentioned above.
The action bar itself is pointless outside of general design. The graphic for mine is completely black so its blank in game. The hotkey slots are all boarded by the button pieces as well are the window selector buttons so the ones drawn on the action bar graphic mean nothing and you cant even see them behind the default button boarders. So if you plan to make a new bar graphic just take the general size of the button area into account don't worry about making button spaces on the graphic.