Writing a sponsership letter for tourist visa to UK

I would be the sponser for my very close friend who is coming to UK as tourist very soon.
Can u kindly help me out how to write a formal letter showing that I will fully support her on her journey to UK and while she live in here for six months please?
I just want to add some more informations that I can show my bank account details,my status document in UK and other additional documents that may require in my letter.

many thanks,
Wine