Page iii of the North Atlantic MNPS Airspace Operations Manual, 9th edition:
‘Pilots MUST NOT fly across the North Atlantic within MNPS Airspace, nor at flight levels designated as RVSM Airspace, unless they are in possession of the appropriate Approval(s) issued by the State of Registry or the State of the Operator….. Within NAT MNPS Airspace a formal Approval Process by the State of Registry of the aircraft or the State of the Operator ensures that aircraft meet defined MNPS Standards and that appropriate crew procedures and training have been adopted.’
Your authority will probably require you to attend a course at an establishment such as FlightSafety and to produce for their inspection a briefing document outlining your company’s North Atlantic SOPs before approving operations.
From experience ATC will from time to time ask for your approval number before you enter North Atlantic airspace.