Surely I don't need to explain to how you how to open a png file in your favorite image editor, and make sprites, right?
Well, there's actually a little bit of more information you should know before you spend your hard work on sprites.

First, It's important to know that the outer pixels of sprites go completely unused in IIDX. So, for example, in this note skin,

the only pixels that will get used by the game are the blue and white ones. The red pixels that border the sprite are not visible at all to the player.

You should also know that your sprites MUST be saved as PNG, you can not use JPEG, BMP, fuckin WEBP, only PNG.