I love this exercise. As I say in the article, it is deceptively simple. It looks easy until you sing it with only one reference pitch and then check yourself at the end. The second exercise is even more challenging to get the modulations correct. Of course, as with most things, it gets easier with time and practice.
Once you can do it with Autumn Leaves, try it on All The Things You Are, What Is This Thing Called Love, Have You Met Miss Jones, etc. It can become a great tool to learn tunes.