I did a brief cover letter, and threw it in the mail: This is what I used (lifted from someone else, on Calguns, I think)
Stick it in an envelope with the form, addressed to the CLEO and drop it in the mail.
(Date)
(Your Name)
(Your Address)
(Your Phone Number)
(CLEO's Name)
(CLEO's Mailing Address)
Dear Chief (CLEO's last name):
Enclosed please find, as per federal regulation, a copy of my ATF Form 7CR. You are not required to take any action with respect to the enclosed form. If I can be of any service, or if there are any questions, please do not hesitate to contact me at the address above.
Sincerely,
(Your Signature)
(Your Name)
Stick it in an envelope with the form, addressed to the CLEO and drop it in the mail.