Back to the gown. To attatch the skirts to the bodice you first have to pleat them down to a size that will actually fit into the opening on the bodice bottom. I usually do this by putting the bodice on the dress dummy, sitting down in front of it and then simply start pleating. I've got an almost uncanny ability to pleat exactly the size I need with very little re-do. Must be a gift.
Each individual pleat needs to have its own pin. Once they are all pinned, and you check that they fit perfectly into the opening, it can be pinned to the lining fabric only. I usually press up the edge of the bodice along the stitching line. It makes an easy guide for sewing and a nice clean seam. Sew the bodice onto the skirts by using a back stitch, about 10 inches to the inch IF you are hand sewing. I machine sewed this into place.
Sew the skirts onto the bodice lining. Once that is done you need to hand stitch the bodice onto the skirts. This can only be done, IMO, while the gown is on the dress dummy. The point of the back of the bodice needs to be sewn down ontop the skirts several inches. If you do this with the gown laying down it will not lay flat. Pin along the bodice edge and sew onto the skirts using a running stitch. Remember not to sew through the lining.
The skirts are now attatched to the bodice. To finish up the gown hand sew the side seams and the hem.