In the first stanza of "Song VII," the speaker imagines the poem that he's writing as a woman who is taking off all her "adornments" and "decoration" (1-2). The speaker thinks this is for the best. That's because fancy poetry (which is equated with a woman all dressed up) distracts the speaker from really being able to access the divine. The speaker wants communion with God (whom he addresses directly), and he realizes that he can only have that communion if his poetry is simple and is written straight from the heart.
In the second stanza, the speaker compares his poetic abilities to those of God. God, to him, is a "master poet." Again, he addresses God directly and tells him that he feels pretty pathetic when he holds up his own poetic powers to God's. (Who wouldn't?) In this stanza the speaker also uses the metaphor of music. He says that all he can do, being the measly little human being that he is, is to make his life "simple and straight" like a "flute," so that God can "fill" him with music (8-9). In this sense, the speaker suggests that he really can't make poetry. Only God can make poetry through the speaker, in the same way that a musician makes music through an instrument.