You should begin with a chart that separates both similarities and differences. Read both articles and annotate everything you notice on to the chart. You could then begin drafting all your work and revise.
The dark cloud was like a stain of darkness over the wide-open plain.
(Is that too advanced?)
There was a red stain on the boy's shirt as he stood in the grassy plain.
(That's a bit simpler.)
No, I don't think it is a rhetorical device
