how to issue a section 8

Possession - Section 8 and Section 21

i have read your guidelines but i am still a little confused. My tenant has been in the property for 3 years, her rents was increased by us in Dec 08, she was slightly in arrears at the time, she has paid 1 small payment since then, most of her rent in from the Dss. She is now over the equivalent of 8 weeks in arrears. On reading the guidelines it says that you cannot issuse a section 8 under an assured shorthold agreement if the 6 months have elapsed. Do i just put it in writting to her or there a form.

please help.

many thanks