I prefer the overhand knot with both top and bottom held and tied together as one.
A hint though---I just made a Hannah Montana pillow kit from JoAnns for my great niece and I learned one thing the hard way----tie the knots on the sides that DO NOT stretch first---then on the stretchy sides last. Especially if you are going to insert a pillow form.
I did not realize this until too late and it was not easy.
If you are making your own, make sure your strips are at least 4 inches long or longer.
Other than that, they are simple and fun.