Help with modding - adding monsters to new areas

[Paarthurnax99]

So I'm in the process of modding/creating some custom quests and I'm having trouble with adding existing monsters to new areas - for example I'm trying to add Barroth to the Hoarfrost reach to make a Jade Barroth quest but I can't seem to get it to work. I know it's possible because I've seen loads of mods do it and I have an idea on how to do it but it's not working and I've been on it all day. My problem is that, I either end up with an endless loading screen or the monster is invisible and won't move so like the icon will be on the map and the quest will run but the monster will be invisible and not move. I've looked at various tutorials and even using some mods as templates to go along but they never seem to work. Why is the monster invisible? is it something to do with missing collisions? Any tips/advice on how I can get these working? It seems all fairly straight forward because I've seen lots of mods be able to do it. Would be much appreciated :)