I made this guide mainly for those who have already a basic knowledge in photo editing. Like I said it isn't very difficult and other people can teach it much better then I could.
Sorry I don't know Gimp tutorials, but their site has a tutorial section or you can search on youtube for something different.
*use the lasso tool to refine the rough selection of the magic wand (sorry seems like I forgot to finish the sentence)
About editing in Gimp: its been more than a year since I last used Gimp, but I recall correctly you can directly apply adjustment on the layer you are working on (so the background and the character), you don't need an adjustment layer for it, but it's discussed for sure in some tutorial.
Hope I didn't totally missed your point
Artist for Senjougahara picture: Watanabe Akio
Stock image from Pexels.