The easiest way I have done it so far is to bake a round cake.. depending on what size you need either use the mini ball pan or the regular ball pan.. then use cardboard to make the top board and cover with fondant that matches the ball pan/hat part.. I hope that makes sense.
I used the Wilton Wonder Mold and it worked really well. The top part of the hat I used a piece of cardboard covered in fondant. For the tassel, I actual went to the bookstore and brought a tassel just like the one my niece would have on her cap. It was a nice extra for her to keep.