A Canadian citizen with a complete file can apply at a port of entry and, if admitted, may begin the authorized employment, while an I-129 petition adds USCIS processing time. Complete the support letter and credential evidence before choosing and scheduling the procedure.
Sequence the paperwork around the installation season
Work backwards from the first installation date. The transcript request and the employer's rewritten letter are the slow items; a college may take weeks to issue official transcripts, and a distributor's human resources team may need time to name a supervising engineer and revise a title. Once those exist, a port-of-entry application allows a same-day decision for a Canadian citizen, though a refusal at the border means going home and repairing the file.
An I-129 petition gives a written approval before travel and is preferable when the profession fit is arguable, at the cost of processing time or a premium processing fee. Do not let the employer schedule customer visits on the assumption of admission; a technologist who arrives to work without TN admission is working without authorization. Book the installation calendar after admission, not before.