Performance and Lesson for Study No.25 in D Major (Andante Cantabile), Op.60 by Fernando Sor (1778–1839). This is from my PDF or hardcopy sheet music or tab edition for classical guitar: 25 Progressive Studies, Op.60 by Fernando Sor.
Fernando Sor’s Op.60 is one of the most famous etude collections in the classical guitar repertoire. Each study covers a different pedagogical concept but the primary goal of the collection as a whole is the ability to play legato across various textures.
Here’s a lesson on harmonics in general from my technique book.
I still place this study at approximately the grade 7 level depending on your experiences with harmonics. Here’s the YouTube Link for this lesson if you want to watch it there.