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.