ICAO produces a lot of information about international operations. Start with ICAO Annex 2 and Annex 6 Part Two.

ICAO member states issue Aeronautical Information Publications (AIPs) to inform foreign operators how to fly in their territories legally. When researching a new country to fly into, the sections of high priority are…

Crossing the Atlantic along the Blue Spruce Routes is best accomplished between FL200 and FL280. Below FL200, you start to lose VHF communication and radar coverage. At FL290 and above, additional navigation, communication, and surveillance equipment requirements and authorizations (LOAs) are needed to operate legally. 

The Blue Spruce routes are an option for all aircraft - old, new, broken, or small. When you need an exemption, Blue Spruce it.

The North Atlantic Airspace has specific documentation and record-keeping requirements. Procedural differences for both normal and contingency operations make a NAT crossing unique from domestic operations. NAT Doc 007 is the primary source of information. Here is a summarized guide.

A Master Document is a detailed flight plan and a working record of your flight. It is required when crossing the NAT, verifies you're on track and burn, provides a backup dead-reckoning means of navigation, and is your primary evidence for showing an inquisitive inspector you know what you're doing over the ocean.