Read the sentence:
since you are writing a poem, how will you use metaphors?
identify the independent clause

The independant clause mean the simple subject which says that you can read a part of a sentence that makes a lot of sense. So in this sentence "Since you are writing a poem, how will you use metaphors?"

The dependent clause is the one that makes no sense while being alone. 

In this sentence the dependent clause is Since you are writing a poem.
It makes no sense all alone in one sentence.

Now for the actual question answer is the independent clause. 

The independent clause is how will you use metaphors?

I think that is the independent clause because it really makes sense even though it's a part of the question.