In order to make GUIs you can copy and paste my pack named mcguis and edit it on your likings. In order to create custom GUIs you can use the JavaAPI or install a plugin like ChestCommands. If you are using a plugin like ChestCommands you have to name the GUI like this:\
This tells ItemsAdder to replace the GUI name with a -16 offset special character that will shift the main_menu texture of 16 pixels back.

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Example addon
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