Begin with whichever outside party the file needs most: the designation body, the credential evaluator, or the employer's own drafting process. The border step is short; everything before it is not.
List the outside parties, then write to them
If the file rests on a professional designation, ask the issuing body now what confirmation of standing it provides and how long that takes to obtain. If it rests on a foreign degree, commission the evaluation and the certified translation immediately, because both are measured in weeks and neither can be accelerated by good intentions. The employer letter usually needs two or three drafts before the duties read as professional accounting with a stated period, and each round costs days. Only then does the application itself matter, and at a port of entry it is decided the same day. Where the employer files a petition instead, add the current published processing time. Do not resign a Canadian position until the qualification evidence is physically in hand.
Hypothetical example: a landscape architect is expected to start a design season after a municipal approval is issued in the United States. The first review should make a reverse calendar: credential retrieval, translation or evaluation if needed, employer-letter approval, travel date, and a reserve for correcting a factual inconsistency. The dated offer and the professional credential establish the key milestones; a verbal start date does not. Decide early whether the employer needs petition-based predictability or whether an application on entry suits the business schedule. Canadians can ordinarily seek TN classification directly when eligible, while an employer may choose Form I-129 with USCIS. The avoidable mistake is scheduling a resignation before the documents that support the classification have arrived and been checked together.