I'm wanting to be able to adopt a child into my custom made home. How would I go doing so? Add a marker to my house or something? I've never actually adopted a child yet, so I have no idea how it works.

I'm not sure either, but with the 1.8CK update came the HF sources, maybe they'll help?

Ok, it looks kinda complex, but seems to revolve around the BYOHRelationshipAdoptableFaction faction

I already have the child that's adoptable I want to adopt, I need to get her in my home that's not a vanilla Hearthfire home.

If its similar to followers and spouses, the clue might lie in dialogue, not just factions. But the script will be unreadable until we get the sources. Worth a look though.