Edit: I should clarify, I meant to test things without activitypub. As the other poster mentioned you would need a valid hostname and valid certs for federation testing. Though it might be possible to run multiple local kbins that trust each other, not sure there is going to be much information on that
Someone just went through this themselves in the matrix kbin development channel so you could also potentially find help there if you're having issues. One thing that came up during that is https://codeberg.org/Kbin/kbin-core/pulls/132 has a bunch of changes to those docs which might make it easier