START HERE:
http://www.cic.gc.ca/english/visit/tourist.asp
this is a link for tourist visa application. You can either apply online or you can apply through paper application.
Where to submit the application:
online or
http://www.vfsglobal.ca/Canada/Philippines/
Is your fiancé a PR already? if yes, then both of you can write the invitation letter for her mom. If not, then yes, you are the one who will write the letter. You have to gather your COE, NOA, T4, payslips, bank account statement.. etc. If you need a sample invite letter, please check my previous posts.
Also, is your future mother in law still working? she also has to show ties in the Philippines like, job, house under her name , bank statement, if not then on your letter of invitation, you have to mention that you are the one who is responsible for the expenses etc.
I hope this helps!