Well you may already know this. use import to open mod files then save them in ms3d format. Then when you have 2 ships you want to use parts from both. Open the 1st one. Then click file/merge. Locate the 2nd ship in ms3d format.
btw 1st move the 1st ship 20 or so in any direction so the 2nd ship isn't right on top of it.
From here you can select and delete the parts of the ships you do not need. Then move them around to combine.
I can't think of a tutorial off hand but I am sure someone knows of 1.
Good luck.