Whats the best way to do sit ups?

Don't waste any energy coming down. When you're at the top just "fall" down to your back, catch yourself before your entire body is on the mat, and spring back up. Remember that your head doesn't have to be on the mat, you just need your shoulders so curl your head forward and try to be as compact as possible.

Cpl, I’ve never really given it much thought. If I had to give you an answer I would tell you to exhale pulling yourself up. It’s just like going to gym. When you lift weights you exhale during the rep and breath in between. If that makes any sense. On a side note, you should be exercising daily and keeping your physical readiness at the standard. It takes the worry out of worrying about passing your APFT. Good luck.

Sit-ups arente usually hard to grade. Just come up as far as possible and keep your hands in place. If you knock out one Situp every two seconds you'll be fine