theres a Forging 101 on it, but a better way is, surround your object with double boxes, put a door flat in the middle, then touch your box and save and quit
Or instead of surrounding the objects with Boxes, you could use walls too. Thats the way I did it seeing how I couldn't spare much room to do it. Also, I never needed to save and quit, although many people do. I usually just saved after I did it, just in case something went wrong.