Begin when the employer can explain the actual temporary assignment, before a travel date is announced.
Let the evidence set the schedule
Work backwards from the date the applicant is expected on site. The two long items usually sit outside the employer's control: a provincial regulator or registrar issuing a letter of standing, and a credential evaluation for a degree earned outside Canada or the United States. Both run in weeks, so they start first.
The engagement letter comes next, and it must exist before travel, because a prearranged arrangement cannot be assembled at the counter. Then pick the route. A Canadian citizen may present a complete package to a CBP officer at a port of entry or preclearance location and receive a decision that day, with no visa stamp obtained beforehand; the alternative is the employer filing Form I-129 with USCIS and waiting for adjudication.
The border route is faster but offers no second chance to repair a thin file, and a refusal stays on the record. Neither route fixes an unclear role description. After admission the authorised period is finite and tied to the described engagement, so plan an extension or departure before it expires.
A material change in Canadian or Mexican citizenship, the listed profession, the credential, the prearrangement or the self-employment bar is a question on the day it happens, not at renewal. Where a deadline is tempting an incomplete filing, ask a U.S.-licensed immigration lawyer first.