Just made my first ever mod, a small farm house down by the river. Is there a tutorial or anything how to get your followers to follow you into a created house. Thanks!

Not sure, but the first thing I would do is look at an ordinary game world door. Companions follow through them, so maybe looking at their settings might give a clue? Also check the forums/tutorials for the ESCS and GECK. They've been around a while now, so may have info on this. Seeing as they are effectively the same as the CK, except for tweaks, Any info should be directly transferrable. Hope this helps!

You might need to establish a (correct) navmesh within the house and outside it. Otherwise NPC's cannot follow you inside.

Yep, I gotta wait for a proper tutorial to learn how to do that. Wow, never knew this much work went into making a mod. I just gained a crap load of respect for some of these guys and gals.

Maybe this one will do? http://www.creationkit.com/Bethesda_Tutorial_Navmesh

I'm waiting for the vid. A lot more easy for me to learn that way. Keep up the great work emma. I miss Vilja!

Navmeshing is literally the easiest thing in the world.

I love it. Except when I screw myself and make giant maps. Then I hate it.

Keep in mind that the navmesh can be buggy as well. Any user-created navmeshes (interior or exterior) in ESP format will cause NPCs to sometimes 'forget' their pathing in those areas if the player goes to other places and then comes back. You can see it in more detail in this video:
http://www.youtube.com/watch?v=RlNBVOcuW3M

I made an extra room for Breezehome and added a door that teleports you to it. I've done the navmesh in the new room, added portals for the doors, all of that, it works fine. But, for some reason, Barbas doesn't follow me into the new room. Any ideas why?

Edit: Ah, was error on my end. Forgot to finalize the mesh.

Edit2: Oh and @OP, check the creationkit.com page for Navmesh. I used that. It basically automatically creates the Navmesh and then you just edit it a bit and fix errors if there are any and when you're done, finalize it and that's it. It's actually quite easy to do Navmesh in rooms and smaller areas.

You need to finalize the navmesh both inside the room and outside where the door is, and make sure that one navmesh triangle at each door (inner and outer) turns green; this means that the door is properly set up for NPC use. If both sides are green and your companion still doesn't follow you in, it might be because of the navmesh bug that I mentioned.