Have the factual position description, pay, and intended period settled, along with the applicant's qualification and identity records. Any material difference between the underlying offer and the support letter should be resolved before the letter is finalized.
Let each party confirm only what it actually knows
The applicant can supply credential and identity records, while the employer is the appropriate source for confirming the actual duties, reporting structure, and pay. If a staffing agency and an end client describe the assignment differently, ask them directly to reconcile the discrepancy rather than picking whichever version looks better. Keep dated versions of every document and identify which one is current before submission.
A credential evaluation from a recognised service states what the foreign qualification is equivalent to in United States terms, and a certified translation renders the parchment and transcript in full, with the translator's attestation of competence and completeness attached. A payroll register shows which entity actually compensates the professional; a signed services agreement shows which one directs the work; a degree transcript or an entry in a licensing body's register shows whether the qualification attached to the listed profession is met on its face.