This is your bare minimum sample form but you can add some frills if you like:
Canada immigration
address
date, town, province, country
withdrawal of citizenship application
I, (name) wish to withdraw my application for citizenship.
(here you can put your reasons why but that is optional)
Sincerely yours
________________________________
(name printed, sign on the line above)
Date of birth: (write your dob)
PR card number: (write your PR card number)