The multiple-choice section is scanned optically. Fill the bubbles completely with a 2B pencil. A dot or a tick will not register. Extending a mark outside the bubble will not register either.
The free-response section is graded by humans working from scanned images. Write inside the printed boxes. Anything outside the box is invisible to the grader.
You may not use correction fluid or tape on the answer sheet. If you make a mistake, cross it out cleanly and rewrite. Do not scribble.
Leave the last five minutes of each subject strictly for transferring answers from the question booklet to the answer sheet if you have been working on scratch. This is when the most preventable errors happen.