Confirm the profession and credential first, then obtain the signed support letter, then decide the venue, and only then give notice and sign a lease. A port-of-entry application can be made days before the start date; a petition needs the USCIS processing time built in.
Nothing irreversible until the letter is signed
The mistake a Shannon applicant most often regrets is resigning on the strength of an offer before the support letter exists. The safe sequence starts with a written confirmation that the duties fit a listed profession and the credential limb is satisfied; for a diploma-plus-experience case that includes collecting the employer letters, which can take weeks if a former employer is slow. Next comes the U.S.
employer's signed support letter, then the venue decision. Hypothetical example: a systems analyst with a clean file and a Canadian passport can plan to apply at a port of entry a few days before a start date, keeping her Québec notice period short; the same analyst with a prior U.S. refusal should have her employer file Form I-129, with premium processing if the calendar is tight, and give notice only on approval.
Sign the Colorado lease after the status is granted. Diarize the three-year expiry and the extension well before it.