You must spawn the resulting clothing and name it something like new clothing or simply a number like "1".  This clothing will appear right over the original dress that you performed the fitting on in the fitting room.  You will need to delete your original dress be it conforming or dynamic; this new clothing replaces the original one (you will retain all your morphs, textures etc).  If you find that you have some poke thrus etc with your new garment, bring it back into the fitting room repeat, new name and back in the Pose room delete the new older garment (You should have only one garment when finished).

Then do what Hborre instructed you to do.  Jan