As I said above, the hourglass is the most difficult of the standard shapes to build. And the hardest part is the shaping of the sides. The easiest way to do that is to hot-water soak the sides for a couple hours to soften them. Then bend the side pieces into a jig which then holds them while they dry. I've attached a couple pix of simple peg jigs. Jig 1 is almost too simple but will work just fine. I prefer to use 1/2" dowels in a 3/4 plywood or chipboard base, similar to Jig 2 but without as many dowels.
Make the dowels about 1/8" shorter than the height of the sides. That way, you leave the sides on the jib and use it to hold things while you add the head and tail blocks and the bottom and glue them into place -- making what's known as the carcass.
Then remove the carcass from the jig while you make the tuning head, and the top/fretboard assembly.