Here are a few helpful sites I found on the net for those who plan to do Avani Avittam/ Gayathri Japam by themselves and are looking for pointers to the procedure for the same:
  1. http://www.trsiyengar.com/id62.shtml
  2. http://www.vadhyar.com/Upakarma%20Mantram.php
  3. http://www.prapatti.com/
PS: I am not connected with these web sites and do not vouche for the authenticity of the information furnished in them