I'm doing a lot of humanities subjects, and I think that this one probably depends on your teacher, and the format of the SAC. My Lit/Revs SACs are pretty different from my Interpol ones. My teacher likes to switch things up a bit, so sometimes we have extended response questions (usually 1.5 pages for each question, and we do three questions in two periods), sometimes we do an essay, sometimes we do a report.
If it's an essay, I usually just establish a contention in my introduction, do a few body paragraphs about each point I'm trying to make, and sum it all up. Pretty basic. For the extended response questions, I just cram in everything that I've learnt about that subject. Don't make the same mistake I did, though - he didn't take marks off for it, but make sure you organise your ideas into paragraphs. I have a tendency to ramble. I usually don't have a contention for extended response questions, although I always make sure to provide arguments for both sides. For the written report, I tried to be as impartial as possible, except for the part of the SAC that asked me to provide an opinion. Similar format to the extended response question.
Is this your first SAC? It's Semester 2, so I was just wondering if you've done any previous Interpol SACs. If you'd like, I can post my past SACs here for you. My friend wanted to read mine, so I've taken a photo of them. Not the best quality, and my handwriting is dodgy, but if you want to see them, let me know. Honestly though, the structure of my Interpol writing is really bad. I just ramble. But he still gives me A+'s because I remember absolutely everything he's said about the subject, and write it all down. Can't fail that way.