List the matters you communicated to those charged with governance, score each on four dimensions from 1 to 5 and see which score high enough to be considered for key audit matters. The cut-off is yours.
SA 701 selects key audit matters from the matters communicated to those charged with governance, taking into account areas of higher assessed risk, significant auditor judgement and significant events or transactions. Of those, the matters that required the most attention are key audit matters. The scoring and cut-off here are a drafting aid and not part of the standard. Check the current text of SA 701 and whether it applies to the audit.
During the audit you communicate many matters to those charged with governance. Key audit matters are the ones that mattered most, so the list is usually short. Scoring each matter on risk, judgement, significant events and effort gives you a transparent starting order. Record why a high-scoring matter was left out, and why a matter was kept, because the reasoning is what a reviewer asks about.
Each matter is scored 1 to 5 on four dimensions drawn from the factors SA 701 asks the auditor to consider. The total runs from 4 to 20.
A matter is a candidate if it was communicated to those charged with governance and its total is at or above the cut-off you set. Others are shown with their score but not shortlisted.
The result is a drafting aid. The auditor decides which matters were of most significance and how to describe them.
Four matters scored on risk, judgement, event and effort.
