I discovered this exercise years ago in an excellent book called The Practice of Poetry, edited by Robin Behn and Chase Twichell, while teaching a creative writing class in Texas. It really worked as a way to get the class thinking about and using comparisons in their writing. If you're having trouble with the April challenge, this would be one place to start. (However, there's no reason to limit yourself to the challenge: if you end up writing a poem that doesn't fulfill that purpose, post it in the general submissions form.)
It's helpful for fiction writers to think about figurative language, since we all use it, consciously or not. Consider April a good month to practice using the tools -- such as metaphors and similes -- that poets use all the time.