Preparation should begin as soon as the turnaround schedule is confirmed, since port-of-entry processing depends on complete evidence being ready on the day of travel, not assembled after arrival.
Build the evidence file before the turnaround schedule is finalized
Because TN status for Canadian citizens is often adjudicated at the port of entry itself, there is no separate months-long visa processing queue to plan around, but that also means there is no room to assemble evidence after arrival. Once a turnaround date is confirmed with the U.S.
facility, the support letter, credential evidence and duty description should be finalized well before travel, so the officer reviewing the case at entry has a complete file rather than a promise that documents will follow. For an employer bringing the same engineer back for a second or third assignment later in the year, each new turnaround should trigger its own preparation cycle rather than reusing a stale letter from an earlier entry, since the specific project scope and dates need to match what is actually happening on that trip.