Also, I was wondering if there is a tutorial for importing custom characters
There was, but the process has been simplified from what it was. Dev is working on a tutorial for generating images with art style similar to the game's art style. Hopefully, it will also have an updated tutorial for custom characters with it.
For now, you mostly just need the image of your character you are going to use for the game. If you don't have one, have an AI generate one for you using Civit.AI or Perchance. Preferably a full-body portrait in 2:3, around 2k resolution though it can be less, with a neutral pose, and a simple plain background for editing in GIMP.
Look online for a free 2d to 3d model converter to make a 3d mode of your character. You should get a .glb you can import during character creation for your directionals.
Make a copy of the full image and resize it to 32x32 for the small no direction image, if you care about that.
Edit the full body image in GIMP, add alpha channel to image and remove background to make it transparent. Save and Export as a .PNG, then import the image during character creation. That should be it for the minimum requirements.
I know that probably won't answer all your questions, but at some point you'll have to learn to figure it out on your own. Ask ChatGPT for more specific help like prompt hierarchy or how to remove background from an image. I know I've done that to figure the small things out myself.