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I know this is going to sound extremely weird, but I have never mailed a letter in my life before.

I'm interested in mailing to a celebrity (Shay Mitchell) in the USA, from Canada.

Do I put Canadian stamps on the envelope and USA stamps on the SASE? How many should I put?

Yep,you are correct put Canada stamps on the envelope you sent to the the celebrity and us stamp on the sase:) first don't put permanent Canada stamp on your envelope you sent since it's for local only international mail is $1.8 about,but for us I'm not sure;)
Go and ask more info in the post office there are lots everywhere,I believe you know where they are,staples,shoppers

It all depends on the size of the envelope and the weight. The Canadian stamps you are going to specifically need are the ones that have a doe on the stamp.

Stamps from Canada to USA cost $1.05 each
This link on the Canada Post website might help you, it's a link to US stamps:
http://www.canadapost.ca/shop/Postage/S ... ution=e1s1

So long as they add up to the correct amount, almost any US stamp can be used for international postage. They do not have to say 'Global.' (You cannot use stamps marked 'Nonprofit' or 'Postage Due' or "For Government Use,' for example.)