Begin once the employer can describe the real work, before fixing a U.S. start date around the job title. The main preparation dependency may be obtaining a substantive manager description or academic records, not filling out a form. A recruiting deadline does not establish eligibility or processing speed.
Set a category decision before final drafting
Ask the manager to confirm responsibilities and the applicant to collect the relevant education evidence in parallel. Once those facts are available, determine whether a listed profession can be supported. If the duties are still changing during hiring, update the assessment before signing the final letter.
Leave room for another immigration strategy if TN is unsuitable, and do not perform U.S. work on the assumption that a technical offer will be approved. The failure mode is a start date fixed before the category is settled, which converts an open question into a deadline.
Once a resignation is tendered and a lease signed, the pressure runs toward making the role appear to fit rather than assessing whether it does. The earliest signal is a request to describe the duties in whichever way is most helpful. Set the category decision as an explicit gate before final drafting, and tell the employer that the date follows the decision.
If the duties are still moving during hiring, that is itself a finding worth recording. No U.S. work should begin on the assumption that a technical offer will be approved.