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...

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.