Do you mean, what to write about spouse's arrival in Canada? I don't remember what we said, because I was already in Canada on spouse OWP before that.
In the letter of explanation, my wife simply explained what she is applying for (PGWP with a spouse OWP for me). She just explained that she was applying on the very same day that she graduated, because otherwise she wouldn't have been allowed to continue working (because she stopped being a student). I doubt our letter of explanation would be of any use to you. The letter of explanation should simply tell them what you are applying for and any specific thing you wish to emphasize. But stating clearly, "I am applying for X and for Y for my spouse" is how you should start, I believe. List where you are graduating from and any other specific circumstance that you wish to emphasize. For example, if your spouse is abroad, emphasize that you are applying for SOWP, too, together with PGWP. That will be a different application, but it helps them to sort things out. Always make things as clear as possible and easy for administrators to figure out what you are asking for! That is my main advice. Good luck with your permits!