Go to an art supply store and buy a staple gun, also a pair of canvas pliers- these are different to other pliers because they grip the canvas with a flat, wide head. Buy a wooden support frame.
Put the painting face down onto a towel on a sturdy table, so as to not damage the face of the painting.
Staple once in the middle of one side, at the back of the frame- and then repeat in the middle of the opposite side. What I mean, is, lets say the edges of the frame are north/south, east/west move from North to South then from East to West. At the beginning, you dont need to stretch the canvas much, but you might need to redo the original first couple of staples as the tension increases.
Repeat this for the other edges of the painting. Keep working- North/South East/West, stapling from the middle, out from the middle, to the corners, making sure you are keeping all the tension even- this is the tricky thing.
When you get to the corners, you need to fold the canvas so that you can staple it and keep it flat. When you buy the frame, you can also buy-or it may come with the frame, small wedges of wood that can be gently pushed into the sides of the frame so that it opens a tiny bit and increases the tension of the canvas..
Good Luck!! 
PS- Im pretty sure utube has these instructions...
