The best way to submit an immigration letter for a friend to the relevant authorities is as part of the applicant's overall application package, either in hard copy or online, depending on the application process. If the letter is being submitted as a hard copy, it should be printed on high-quality paper, signed by the writer, and sent in a sealed envelope. If the application process requires online submission, the letter should be scanned and saved as a PDF file before uploading to the appropriate website.