If you do not know all of the information, leave it out. As your life unfolds, you will, overtime, pick up other bits and pieces of information and you can add it in. Hence, as you grow learning Maori, your pepeha will grow with you.
Make sure, if you have parents from two different places, that each side fits!!! For example, if your father is from Ngati Porou (East Coast) and your mother is from Waikato, you would not say Ko Waikato toku awa, Ko Ngati Porou toku iwi as Waikato is not the awa for Ngati Porou.
When naming your grandparents and parents, put your grandfather / father first then your grandmother / mother second. Make sure that when you name the women, you use their maiden name as the maiden name is the connection to their blood lines.