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UndertaleModTool
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After you've made a couple small games on your own - without being dependent on tutorials that handhold you through the process, that is - you should be able to pretty easily read and modify the decompiled code generated by UndertaleModTool after opening Undertale's/Deltarune's data file in it: https://github.com/krzys-h/UndertaleModTool
Assuming you don't have any prior programming experience, since your goal is Undertale modding it might be best to start by learning how to use GameMaker (with the GML programming language, not the drag-and-drop): https://gamemaker.io