Mark the envelope and the cover letter with the type of submission. For example, Original Submission, Application to Register Permanent Residence or Adjust Status (Form I-485). Mark the envelope and the cover letter with the form number.
You may always download a copy of the Form I-90 PDF from the USCIS website.
Immigration forms can be ordered directly from the USCIS website or via telephone through the USCIS forms request line at 1-800-870-3676.