You could give it a wash to make sure there isn't any coating of anything on it, then iron it flat. I have done well on cotton canvas with freezer paper stencils, because then you can iron the pattern on to get good adhesion. Also, in case you are heavy in your application of paint, you may want to put some scrap paper/newspaper in the bag when painting, to make sure there is no bleeding of paint through to the opposite side. Also to ensure that the paint doesn't wear off with washing, you may want to add some fabric paint medium to the paint. I think Plaid makes a version of it, and I personally use the Golden brand. Then when it dries, you can set it by ironing, and it will be more flexible and wear better through washing, rather than just plain acrylic.
Here is a tutorial I posted on my blog a few years ago that may be helpful for you.http://onyxnox.blogspot.ca/2011/08/freezer-paper-stencils.html