I wanted to give you an idea of what 'large' means but I didn't really make myself presentable so I just tossed my arm in for scale.
Yeah, I also used a flash for this picture. Go ahead and poke me in the eye for being a bad blogger.
1) Cut the plastic bag so it's roughly the size of your canvas.
For our large canvas we used a trash bag but if you have a monstrous canvas I bet a plastic shower liner would work wonderfully.
** You definitely want to use plastic - not paper - for the paint transfer. Paper will absorb the paint and transfer very little to the canvas.
2) Once the bag is cut down, fold the piece in half and crease it. This makes it easier to eyeball where the center of the canvas and plastic line up.
3) Put paint on the plastic. You don't need a ton - roughly pea sized blobs
4) Fold the plastic in half and massage the paint around.
This blends the colors and ensures that both halves are covered.
5) Open the plastic, flip over, and press onto the canvas THEN pull the plastic off, admire, and let it dry.
Easy-peasy and done in 10 minutes :)