This was my costume for my brother's musician-themed 18th birthday party.
It was pretty simple - I cut out a rectangle of felt with a long vertical piece in the middle to be the neck. Trace around the neck piece and cut out another one, sew them together and then stuff the neck.
Then, cut a million little squares of white fabric. Lay them out in lines and pin them to the felt, then sew across the top of the lines. It looks better if you fold the squares over a little and use two or three at a time.
Then use a bunch of safety pins to join the ends together and attach it to your tshirt. =)