Siwani,

I'll give you a general idea.

Letter addressed to: "U.S Department of State, Non-immigrant Visa Processing Unit".

Subject could be : Request for H1B Visa Stamp (Request of "employer" on behalf of "employee")

First paragraph:

Should mention that you employer is submitting the letter in your support as a request for H1B stamp. Should mention your job title and that they have received the Approval notice (Receipt #...) of your H1B petition on your behalf from USCIS.

The Petitioner:

Could be a few paragraphs about your company.

Terms of Employment:

Should mention your job title, period (3 years) and that it is a temporary position. Should mention your salary and that it is the prevailing wage for similar positions and will not adversely affect the working conditions of similarly employed U.S workers.

As for your interview, take every document you have with you for you never know what they'll ask for. Carry a copy of your LCA submitted to USCIS along with your H1B petition which is the first thing they ask for after your Approval notice. Other documents they might ask for include proof of your F1 status (all I20s), your transcripts and all documents related to your F2 status, pay stubs if you ever worked on OPT, W2 forms if any, proof of your residence in the US, bank statements etc. Keep all related phone nos.handy (your University registrar's office, land lord, bank etc). Documents showing your maintaining legal status while in the US are the most important ones.

H1B visa stamping is considered easy in Canada and Mexico for US degree holders and is rarely denied if you have all the supporting documents. Just in case, bear in mind that if your visa is denied, you'll have to go back to Nepal directly and it's always wise to prepare for the worst.

Good Luck!