depending on your coding experience, you might want to start smaller than a full-fledged tool. i've written a lot of code but only for myself and never for a console. so i did this:
https://gbatemp.net/threads/i-added-a-time-left-time-elapsed-display-to-waithax.454536/
i took an existing program that lots of people use and added a simple but useful feature to it. even with good C knowledge it took most of 2 days from start to finish just to add what ended up being less than 10 lines of code to a not-very-complex program. most of that was figuring out the best of several methods to print to specific places on the screen without any full-on 3DS tutorials available, only sample code.