Extension of visitor stay for mother in canada

[grewal83]

hi. my mother came to visit me in april 2018 and she has got the multiple entry visit visa. The immigration officer stamped her passport for 6 months entry. now she want to stay with me for another couple of months so i am planning to apply for extension of stay.i live here by my self . what reason should i mention in the letter for extension and also what other supporting documents should i or i need to send along.
please help
thanks