Usually finding the right paper to read is the first major challenge, but since your professor gave it to you, it seems like that is not a problem for you right now.

One thing I do first is try to locate where the theorem the paper is referring to as the main result is. It can usually be done by only reading the English and ignoring the formulas, because there would be plain English summaries in the title, abstract, and in the introduction. I would look at the proof without understanding it, and it would mostly refer to some important lemmas or new definitions, and then I would find where those are, and look at the section headings where they appear. While I am scrolling up and down, or flipping through doing that I usually come across something the author talks about at great length (e.g. a symbol repeated many times, or a long proof) that I don't understand why the author spends so much effort on that. Most of the times, that is the new idea of the paper I need to understand first. I of course don't know what it is, so I make a guess, then I start looking at the details to check whether I am understanding the paper correctly.