How do I make a picture collage on a canvas?

[Kourtney]

So I really wanna make a photo collage on a canvas. But how? I already know what tools to use since I've seen videos but none of them have been clear or the most i could find were people using photoshop beforehand and i don't have that. I'm gonna get all my pictures from tumblr (social network/blogging site). Do i send them to walmart and get it printed out on my phone? Do i give them sizes? Also if you have any decorating tips or any small ideas that would be helpful also thanks