Draw a Equilateral triangle. From each of these sides draw another triangle shape making sure the angle is the same degrees for all sides.
Add the curved lip to the top.
Cut the shape out and score down the lines of the triangle in the centre.
Fold along these lines and hook the curved lips around each other.
Now you're done! You can either decorate these or use decorated paper, i've done both.
*EDIT* i've added a diagram of my measurements to help people!!
hope it helps! :)