Which of his own pieces of writing does Poe use to explain his philosophy of composition?

The essay "The Philosophy of Composition" by Edgar Allen Poe was written in 1846 and originally published in Graham's Magazine. In it, Poe details his theory and procedure for writing a piece of literature such as a poem. First of all, he says, length is an important consideration. Literary works should be short if possible, and ideally they should be able to be read in one sitting. A literary work should have unity of effect that leads it to an inevitable conclusion and provokes an emotional response in the reader. The writer should methodically use various methods in order to achieve these objectives.

The piece of writing that Poe uses to explain his philosophy of composition is his poem "The Raven." Poe writes that he chose the subject matter for the poem because death is the most melancholy topic within the universal understanding of humankind, and the death "of a beautiful woman is, unquestionably, the most poetical topic in the world." Poe uses excerpts from "The Raven" to exemplify various literary techniques such as the use of a refrain, rhythm, the establishment of a climax, locale or setting, and the effect of a denouement.

"The Philosophy of Composition" first appeared as a magazine article in 1846. In it, Edgar Allan Poe elaborates the principles that he believes should be used in writing. For Poe, artistic creativity was far superior to factual expository writing.
Although he espouses creativity, he also advocates a methodical approach and casts doubt on revelatory insights. The work he uses to explain the theories in practice is his poem "The Raven." In fact, he says that poem was written to illustrate his theories.
In advocating his methodical approach, Poe identifies subject matter of the type his Romantic poetry and fiction often employ. Chief among these is a dead, beautiful woman, especially as her lover praises and mourns her. As this is precisely the subject of "The Raven," it supports his point well. He also details the process he followed in composing the poem.
For Poe, all the components had to work together in service of the whole, toward one unified effect. There was no room for loose ends. Keeping the piece short helped create that effect. Choosing the desired tone and the words that best convey it was also important; "nevermore" fills that bill in "The Raven."