First, you need to determine the external documents relevant to your quality management system. They may be standards, legislation, agreements, and contracts. You may develop a register of this kind of documents.
Then, someone in your organization has to distill those external documents into a set of applicable requirements that must be implemented and followed.
Then, you must be aware that many external documents issuers don’t know and don’t care about your organization, but your organization has to comply with their requirements set on external documents. This means that they may issue new external documents or revise existing ones, changing requirements, and your organization has must be aware of these changes. So, someone, periodically, has to verify if external documents were created or revised. ISO 9001:2015 does not state a frequency for this verification, each organization has to decide what is the most appropriate, considering risks and economic dynamics.
To make a procedure look for the verbs in the text above (what needs to be done), decide who will be responsible for the checking and with what frequency.