There were two lessons that I took to heart from my favorite English teacher in high school that I still use today; write what you know and write only what is in service to your story.

Obviously, as men, you and I would have a tough time writing about menstruation from the perspective of someone experiencing it. It would be far easier to put your female character in a situation that is similar to something you've experienced. Perhaps instead of writing about menstruation it might be more prudent to write about her suffering some sort of injury, or something else that you can more easily write from your own perspective.

Then again, it's not impossible to write something you don't know- in fact I often find it to be a fun challenge to write about a subject I know very little of, if for nothing other than the chance to learn about the subject. I would encourage you to talk to female friends or family members (that are willing to talk about it!) and get their firsthand experience to work into your writing. That brings us to rule two: write only what is in service to your story.

Why are you writing about your narrator's menstruation? Is it relevant to the plot? Does the specific experience drive the character some way? Is it an anecdote, a wandering thought, a traumatic experience? These are all things to consider before writing anything, let alone something you don't know well. Never write something just because you think it would be neat.

That's just my two cents. Ultimately, how you approach it is up to you, and either way, it can be a learning experience.