Take your time to speak to your teachers about any last concerns or worries about content that you may be stuck on. Ask them questions about anything that you are unsure about.
Get friends to test you on content by giving them cards with info and they ask you a question.
Pretend you have to teach a small group of people the information you are required to learn. Tell a friend or family member information you are learning and teach it to them. This helps get it to sink in.
Set time everyday for yourself where you do only what you want to do that has nothing to do with school or study. It is inportant to remember that there is life outside of school. HSC isn't the end of the world. Think of HSC as a stepping stone. If you miss this step, there are always other stones along the path always leading you forward.
If you are feeling overwhelmed, take a break. There is no point trying to learn when you are not in the right headspace.