One way for a very large hat to fit on the head properly is for a diaphragm to be placed inside. This is accomplished by making a tube using the pattern from the stand of the hat, and sewing a casing in one edge, through which a shoestring or other drawstring will be threaded. This drawstring can be drawn up so that the hat will sit on the crown of the head.
Use the same pattern that you use for the hat form to create the lining of your hat. Cut the lining out, and assemble the lining, first by sewing the seam on the stand and then sewing the stand to the crown.
You may either place the lining in the hat at this point, or you may attach the inner diaphragm to the edge as I did in the photographs. How you assemble the hat is really dependent on the type of edge trimming you want.
Tack the top seam of the lining to the top seam of the hat form so that your stitches do not show. Stitch the lining to the edge of the hat form in a manner that will be visually pleasing when you wear the hat.
Cut out a piece of fabric for the outside of the hat larger than the crown of the hat. Pin the piece of fabric in place and stitch it along the edge with whipstitches to hold it in place.
Cut a piece of hat fabric approximately twice the width of the hat stand, and about 3 inches longer than the hat stand. Place the edge of the fabric along the edge of the crown piece you just whipstitched in place. Stitch a straight line of running stitches or backstitches near the edge of the fabric to hold it in place. Flip the fabric down, fold under the bottom edge, and hem it in place along the edge of the hat frame.
At the midpoint of this piece of fabric, stitch basting stitches vertically and gather the fabric, ruching it. Carefully arrange the fabric and gently pull it toward the opposite side of the hat. Arrange gathers along the hat stand and pin them in place, then whipstitch the raw edge to the hat frame. Arrange fabric on the opposite side of the hat, and pin the ruching in place. Turn under the raw edge of this fabric and hem the edge, attempting to hide your stitching. Tack the ruching in place with small pick stitches.
Thread a drawstring through the casing of the diaphragm. Adjust the gathers so that the hat will sit on top of the head in the proper position. Tie a bow and tuck the drawstring up inside the hat.
You may now add decorations to your hat in any way you like, but especially on the seam line so that you hide it.