There is a tutorial about making a bed cover here:
I haven't watched it, so I don't know if it would be suited for a napkin or not. If it uses dynamics like I think it does, it probably won't be of any use to you as far as the napkin modeling goes.
My advice to you is to take a nurbs plane and fold two corners opposite of each other and fold them underneath. This can be achieved by moving the vertices into their respective places. Just in case you don't know yet, you can display faces, edges, and vertices by holding down the right mouse button on an object.