I couldn't open the translated to English but from the pictures it looks like you make your round one and make a cardboard square which you place the round one on sew behind it to make the round one square. I could be wrong but I will have to go back to the link and see if it will translate to English.
But if that way I thought the pictures mean why could one just glue the back t=of the round yoyo then sew two square yoyo together or even mark the back with a pencil sew lines on the back then sew the two backs (right sides together facing) like doing a cathedral ceiling (mock) type.
http://www.youtube.com/watch?v=3gKvdrexKE4