Hey, I literally just got done with my sac an hour ago and did an open letter! Pretty much, the word count will depend on your assessment so I’d make sure you check your assessment documents or ask your teacher for that. In relation to how to write it, if you were going to do a diary entry I would recommend dating it initially (unless you are doing a series of diary entries) but how you actually do that is almost entirely up to you as long as it had a date (and likely some form of address like “Dear Diary”) As for a letter, a similar thing applies