As far as I remember, you can select it in the art section of the world settings when creating a game. You need to run the automatic1111 webui and write its api. If you have not installed automatic1111 before, this may be a bit complicated for you. And I don't think there will be any tutorial...
oh there are a few games that think about it. I think it would be great, but most 3d characters are ready to use assets. and 2d characters all lookalike with different hair usually