Download Tinke and Crystaltile2. Use them to open the contents of the rom.
Figure it out yourself if it's doable at all.
You could try to hack Wario World 4, it's much easier. You get the palette from the emulator and then open it in Crystaltile2 after opening the rom.
Then you jump between an image editor where you draw your thing and Crystaltile2 using CTRL+C/CTRL+V.
The rest is up to you. I won't say anything more, you figure out the rest yourself