If you have the means, I would recommend watching the Abbott Elementary half of this crossover event (S4E9 “Volunteers”) before watching the It’s Always Sunny in Philadelphia half. The Sunny episode alone does provide a fairly adequate explanation of what is going on, so you will likely not be confused as to what is happening while watching the episode. But the Abbott episode adds a fair amount of backstory that answers some minor questions I had about the Sunny episode. Like I said, I was not overly confused by watching the Sunny episode first, but after finishing the season and then deciding to check out the Abbott episode, I realized that I would have likely had a better overall experience if I had watched both of the crossover episodes in order.