The letter should name the employer, the profession being claimed, the duties, the qualification held, the salary and the intended period. It should be signed by someone who can answer for those facts if asked.
What to ask the employer for
Ask the employer for four things before the letter is drafted: a plain description of the work in the manager's own words, confirmation of which legal entity will pay the salary, the address where the work will be done, and the name of the person who will answer a phone call from an officer. Hypothetical example: a Lévis quality-assurance chemist receives a letter written by an HR generalist in another state that describes the job as laboratory operations. Sent back with a request for specifics, it comes back describing method validation and analytical reporting, which is the substance the chemist listing turns on.
Then supply the employer with your own records: diploma, transcript, any order or association membership, and proof of citizenship. Keep the letter's dates, the offer's dates and any relocation agreement's dates consistent, because a discrepancy in a start date is the easiest inconsistency for an officer to notice.