The beauty of collage as a form of art, is that it can be created out of virtually anything.  There are so many ways you could approach this assignment that I hope you find one that best reflects your own feelings, personal touch, and reflection.

If it was up to me, the first thing I would do is re-read the poem and circle any concrete images that really stuck out.  Some examples that come to mind are:

camp; daybreak; cool fresh air; hospital tent; brown woolen blanket; elderly man so gaunt and grim.

To me, this is a picture of war and death.  But then, I would read the poem again, and try to imagine what tone is actually set.  Is this poem about the grim reality of death?  To me it is not.  Instead, I think it attempts to paint a picture of beauty in the face of death.  So then, I would cling to such images as:

my dear comrade; my child and darling; cheeks yet blooming; the face of Christ himself; dead and divine.

With these two lists in mind, I'd go to a stack of old magazines and cut any pictures that reflected any of the above images or the emotions associated with them.  If I could not find enough from magazines, I might add some of my own with paint.

In such an open-ended creative assignment, the most important thing is going to be your effort and purpose in what ever you create.  If you can explain your intentions, I imagine you will find success in whatever you end up doing.  Good luck.