I'm trying to make a model for a mod I'm making but I think there are some prerequisites to combining items that I'm not doing.

What I usually do is copy, paste and delete blocks and then manipulate the shape. Usually there's no problems but occasionally when I weild the item or hover over it, the game CTD. Do I have to do something before using it in game to prevent that from happening? Something like a 'finalize' before saving and using?

Any help would be appreciated.