So,I wanna make a custom power up for smw I already have the idea of what I want it to be and what I want it to do,the thing is...I don't know how to even make custom power ups,I want to make a cowboy mario power up,it's like a fire flower that shoots forwards,can someone tell me how to make it please?
I recommend you study ASM (or assembly, if you prefer). Has this tutorial, unfortunately I will not be able to help you in anything else, since I do not know anything about ASM. :^)
if you want to study the ASM language completely, have this other tutorial as well. But I recommend that you start with Super Mario World first, since it is easier to understand for beginners, while the other is a bit more complicated.