How should I go about writing a poem from the play Macbeth, using a line from it?
The content of the poem needs to reflect the meaning of the line and represent significance to the play. While it does not have to rhyme, these terms have to be used: look, like, the innocent, flower, but, be, the, serpent, under, it.
Few, if any, can write the poem for you. This will have to fall on your broad shoulders. However, I would point you to one of the last soliloquies of the drama. Act V, scene 5, lines 18-30 provide a wonderful moment where I think that can be used to help construct a poem given the terms that are featured. The idea of the "innocent flower" and the "serpent under it" can be reflected in the idea that life is "a tale/Told by an idiot,/ full of sound and fury,/Signifying nothing." The idea of what life is and what it "looks like" can also be brought out in the earlier lines of the speech, when Macbeth speaks of life as a "brief candle." What is seen as lengthy is actually short, what appears to be is not what is. I think that being able to play around with oppositional ideas in the poem is why this particular section of the drama could be relevant to you finding a line and constructing a poem with these terms based off of it.