Your lawyer should be able to provide you with the employment letter template. Here is an example that I had received. Hope this helps!
PRINT ON COMPANY LETTERHEAD
<DATE>
U.S. Consulate General Montreal
1134 Sainte-Catherine Street West
Montréal, Québec H3B 1H4
RE: Employment Verification for <FULL NAME>
Dear Sir/Madam:
<FULL NAME> has been employed with our company since <DATE> on a full-time basis. We filed an Immigrant Petition for Alien Worker (Form I-140) on behalf of this individual and we continue to support the Form I-140 we filed. We are prepared to offer him a permanent, full-time position as a <TITLE>, earning an annual salary of at least <PAY> at the time this employee’s green card is issued. In this position, this employee will perform the following job duties:
Add job responsibilities
Add job responsibilities
Add job responsibilities
If you have any questions regarding this person’s employment with our company, please contact me at [INSERT TELEPHONE NUMBER].
Sincerely,
[INSERT NAME OF AUTHORIZED AGENT]
[INSERT JOB TITLE]