It should identify the employer, the profession relied on, the specific duties, the qualification the applicant holds, the start date, the length of the assignment and the pay. It is the controlling document, so vague wording costs more than a page of detail.
Write the letter as if the reader knows nothing about the job
Hypothetical example: the dealer's human-resources office offers a one-paragraph template that confirms employment and salary. Ask instead for a letter on company letterhead that names the position, lists the engineering duties in the order they occupy the week, states that a mechanical engineering degree is required and that the applicant holds one, gives the expected duration within the three-year limit for an admission period, and states the compensation. It should also say that the position is temporary and that the applicant will be an employee, not a contractor to their own company. Attach the degree and transcript, and a certified English translation of anything not in English. Keep the letter and the resume consistent: a resume that describes the same person as a sales manager will invite questions the letter cannot answer.
Hypothetical example: a water-treatment design engineer receives a letter that names a salary but omits the project, reporting line, and expected end date. Ask the employer to add a concise description of the design calculations, site responsibilities, supervision, required degree, and temporary assignment period. Then compare the letter with the résumé, diploma, transcript, passport, and any licence. The controlling record is usually the signed employer letter, while the academic records settle whether the stated qualification is real. Do not cure a contradictory résumé by leaving it out; explain an old non-engineering title in a brief cover note. Assemble the final documents in the order an unfamiliar reviewer will need to read them.