The internal audit plan is the detailed agenda of ONE specific audit: which processes will be audited, at what time, by which auditor, against which criteria. Do not confuse it with the audit programme, which is the annual calendar of the whole set — ISO 9001 (9.2.2) requires the programme and the definition of criteria and scope for each audit — the plan is the usual way (ISO 19011) to evidence that, and the external auditor checks both. The plan is sent to the auditees in advance (one week is good practice) to make sure the right people are available and the evidence is at hand.
A good plan defines, for each block of the agenda: the process audited and the owner who has to be present; the criteria (applicable ISO 9001 clauses plus the internal procedures of that process); the assigned auditor — respecting the golden rule of impartiality: nobody audits their own work; and a realistic time slot, with room to dig deeper into findings. Reserve 30 minutes for the opening meeting (to align scope and method) and 30 to 60 minutes for the closing meeting (to present the findings with no surprises afterwards).
The most common mistake in small and mid-sized companies is the shallow plan that turns into a guided tour: two hours to audit "the whole production area" end up producing half a dozen generic questions. It is better to audit fewer processes in depth — following the thread of one real order from intake to shipping, for example — than to skim everything. The tracing technique finds ten times more relevant findings than a memorized questionnaire.
The template below is the plan of a full audit day at a small manufacturer, with two internal auditors. Adjust the duration to the size of your processes.