By the way you need to do it at an immigration office, not at the border crossing.
If you are in Chiang Mai go to the immi office there.
You will need to fill a visa extension application form,
download here
**********************************************************
Also bring with you
2 passport sized photos
Copy of passport photo page
Copy of visa and visa stamp
Copy of your immigration departure card
1900 baht
Depending on where you are they ask for hotel booking and flight out of the country.. I have never been asked though.