Probably the single most important thing is establishing an insurance strategy. Generally the US insurers don't insure out of North America and the European ones don't insure in the US.
Your insurer will probably require you have previous accompanied experience with the crossing (or a suitable briefing - and they will define what suitable is). Your companion or the briefing provider will provide insight into what needs to be done. But from a basic planning perspective there is nothing particularly complex.
Transport Canada apparently used to have a lot of rules on survival equipment - but this is broadly now left to your judgement for a private flight.
Note - the US has just recently introduced a requirement to electronically submit pilot and passenger details prior to departure for a flight into US airspace.