How about a format like this?
Please critisize and make necessary suggestions.
The Embassy of Canada to [Country]
[Address]
[Telephone]
[Date]
Subject: Invitation letter for [Name of guest]
Dear Visa Officer,
My name is [host], born on [DOB], living at “[Address of host]”, with telephone number [host telephone], a/an [host employment], and a [host status].
With this letter I am inviting [Full names of guest], with passport number [guest passport number] born on [guest DOB], and currently living in “[guest address]”, with telephone number [guest telephone]. He/She is my “fiancé(e)”, and will be visiting me in connection with our marriage.
He/She will be coming from “[arrival date - return date], and will be staying at my home [address of host], and I will be taking care of his/her living expenses, throughout his/her stay. He will be leaving Canada on [return date].
Below is attached, a copy of document proving [host] status in Canada.
[concluding polite sentence].
Sincerely, and the likes
[name of host].
Email:[******]
* Please give a clue on client information