Learn all you can about the subject of your film. If your mockumentary concerns kayakers in Finland, read all you can about Finnish culture and kayaking. Then determine what's funny or odd about it and make fun of it in your screenplay.
Develop a dry sense of humor. To succeed, a mockumentary must be subtle and star-convincing, deadpan actors. Write your film in a way that it leaves an uninformed viewer wondering if they're watching a real documentary.
Make people root for you main characters. People were drawn to the clueless characters in "Borat" and "Spinal Tap" because of their likability quotients. Keep you characters dim but affable. Allow the occasional exaggerated bad guy to make people boo, hiss and laugh.
Watch as many famous mockumentaries as you can to learn pacing. The beauty contest spoof "Drop Dead Gorgeous," political satire "Bob Roberts" and folk music mockumentary "A Mighty Wind" will give you ideas on how to write your script.
Keep an authoritative tone, both in dialog and storyline. Your characters should never be in on the joke. The words and settings must seem totally natural.