Find the governing rule
Read the decision letter together with the appeals regulations for your course and stage of study. Check whether the deadline runs from the decision date, notification date or another event, and whether it is measured in calendar or working days.
Keep a clear record
Save the dated decision, email headers and a copy of your submission receipt. If a deadline is close, submit in the required channel with the information available, explain any evidence that will follow and keep proof of submission. Check whether later evidence is permitted.
If the date has passed
Do not assume a late appeal is impossible. Read the rule about extensions or late submissions and explain the reason for delay with supporting evidence. Act promptly, as the university may ask why you did not apply sooner.