You may need to reshape the top (depending on the hat you have). I do this with varying widths of felt spray-glued onto the hat.
I begin covering the top hat from under the brim, in the center back. I have a heavy-duty machine that I can actually sew right onto. I sew on the inside where the side turns into the brim, gathering the fabric slightly as I go (because it will have to spread to fit around the outside of the brim). If you're going to apply this by hand, I recommend running a gathering stitch 1/2" from the fabric edge, gathering slightly, then folding it under along the stitch line and applying carefully to the underside of the brim.
The fabric is then stretched around the brim, then has to be gathered slightly up against the sides. Glue in place. and trim the excess fabric. Glue the top piece on and trim, too (leave about 1/2" hanging over the top edge).
Starting in the center back, fold the fabric strip over and wrap the sides of the top hat. You may need to pull it taut around the top to get the sides to lay flush against the hat - use pins to hold in place as you work. Trim any excess on the bottom.
Go around the top of the hat and tuck in any wayward sequin strands with a pin. You'll also want to secure this seam with a few drops of fabric glue. Fold over the back seam and you can glue or hand-sew it closed (I prefer to sew).
And finally, apply grosgrain ribbon and bow - the ribbon will cover up the fabric from the brim and sides. Click here to see my Blog Entry on making the ribbon and bow.
To adjust the fit you can add layers of felt to the inside and secure with fabric glue. Built the layers up until it fits - but you may not need a strip all the way around.
