The support letter, before the transcript or the licence certificate. Degrees and licences are what they are; the letter is the only document still capable of being rewritten. Ask the employer to correct the job title, the duty description and the statement of the employment relationship while it is still a draft.
Fix the one document that can still be changed
Applicants tend to spend their preparation time gathering credentials and none of it on the document an officer reads first. A transcript cannot be improved. A licence certificate is issued as it is issued.
The employer letter, by contrast, is a draft until it is signed, and it is the document that either connects the applicant to a listed profession or leaves the connection to inference. Ask for changes in specific terms: replace a marketing job title with a description of the professional work, state that the position is prearranged and for a defined period, identify the entity that will pay, and confirm whether the applicant holds any share in that entity. Where the state licensing position is known, say so, and where it is not, do not assert it.
Once the letter is right, the supporting documents simply corroborate it: proof of Canadian citizenship, the degree with a transcript, the provincial licence in good standing, and evidence of any experience the profession's rule requires.