@HiHello7 Oops. So sorry, didn’t see this until just now. This might be too late. 
I was just messaging someone else and telling them that my situation was a little special, because the school didn’t have my ACT scores after I sent them in. My appeal was mostly about that, and basically about how their school had messed up by not getting my scores in time. However, if you were involved in a club, team or community service in high school, talk about your involvement in that activity, and how you’ve grown as a person from the things you’ve learned from there. I think the letter can only be one page, then you can add supporting documents to the packet. Just write a lot on that one page, since that letter is like getting interviewed by the school. Hope this helps. If there’s anything else you want to know, you should message me. I’ll reply a lot quicker then.