A commentary on any piece of text is one's understanding and explanation regarding the piece. Essentially, in offering a commentary, one is offering their response to the piece (known as Reader-Response Criticism).
A commentary on a piece of literature can be completed in the following way.
First, the introduction of the commentary needs to include information specific to the text. In this case, your introduction would give the title ("Swifts"), the name of the author (Ted Hughes), the literary devices used, and the tone/mood.
Second, if the piece is a poem (like "Swifts") the body of the commentary can be split into paragraphs based upon stanzas. Given that "Swifts" has twelve stanzas, this may be too much information for a commentary. Therefore, with longer poems, the main ideas of the poem can be highlighted.
Lastly, the conclusion of the commentary needs to be written. The conclusion includes how the readers feels about the piece, what the piece means (as a whole), and the closing of the piece.