i never really know how to make sprites, I just do it
i mainly use paint, there's this recolouring technique I use which is basically cutting and pasting the whole sheet with different background colours and transparent selection
custom sprites are just the result of me merging sprites together and copying the art style
all i know is how to recolor previously existing sprites. if you want a unikitty sprite, that's probably going to be a custom job which means creating an entirely unique sprite