You can cross certify from an AYSO Intermediate Referee to a USSF Grade 8. The same is true if cross certifying from AYSO Advanced Referee.
1. Print the cross certification form. The form is here: http://www.ayso.org/Libraries/Resources/cross_certification_form.pdf
2. Fill out the bottom of the form with your personal information. Our Section/Area/Region is 2/D/218.
3. Complete the USSF Registration Form. It is available here: http://www.cnra.net/downloads/RegForm/2012RefereeRegistration_eForm.pdf
(all the above USSF info copied from http://www.cnra.net/renew/renew_nocreditcard.html)
4. Mail the cross-certification form, the USSF registration form, and your check to the following:
Duncan Way -- AYSO, 1692 Mangrove Ave, #209, Chico,CA 95926
(Note: do not use the Chicago address on the form.)
After USSF processes your registration, you will receive a USSF badge for the current year, your USSF ID card, and a copy of the Laws of the Game as published by USSF. Every year thereafter, you will (hopefully) receive a post card telling you to register for the next year. You are required to have a current year badge. You can renew online at any time. If you remain a Grade 8, registration is done online.