>>531272ps amateur here.
it all depends.
high contrast and resolution helps to begin with (waifu2x is helpful)
adjusting your brushsize.
refine the edges afterwards maybe with a little smoothness, smart radius or feather but only slightly.
if everything fails: pathes.
you have to track it by hand which is the most work but brings a consistent endresults.
mastering the pentool and its curves brings you independence and freedom.
no idea about gimp.
